Enable the no-var linting rule in src/core/worker.js

This is done automatically with `gulp lint --fix` and the following
manual changes:

```diff
diff --git a/src/core/worker.js b/src/core/worker.js
index aec9c1d39..f88691622 100644
--- a/src/core/worker.js
+++ b/src/core/worker.js
@@ -300,7 +300,7 @@ class WorkerMessageHandler {
         cachedChunks = [];
       };
       const readPromise = new Promise(function (resolve, reject) {
-        var readChunk = function ({ value, done }) {
+        const readChunk = function ({ value, done }) {
           try {
             ensureNotTerminated();
             if (done) {
```

src/core/worker.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint-disable no-var */
 
 import {
   AbortException,
@@ -69,7 +68,7 @@ class WorkerTask {
 
 class WorkerMessageHandler {
   static setup(handler, port) {
-    var testMessageProcessed = false;
+    let testMessageProcessed = false;
     handler.on("test", function wphSetupTest(data) {
       if (testMessageProcessed) {
         return; // we already processed 'test' message once
@@ -100,10 +99,10 @@ class WorkerMessageHandler {
   static createDocumentHandler(docParams, port) {
     // This context is actually holds references on pdfManager and handler,
     // until the latter is destroyed.
-    var pdfManager;
+    let pdfManager;
-    var terminated = false;
+    let terminated = false;
-    var cancelXHRs = null;
+    let cancelXHRs = null;
-    var WorkerTasks = [];
+    const WorkerTasks = [];
     const verbosity = getVerbosityLevel();
 
     const apiVersion = docParams.apiVersion;
@@ -152,10 +151,10 @@ class WorkerMessageHandler {
       }
     }
 
-    var docId = docParams.docId;
+    const docId = docParams.docId;
-    var docBaseUrl = docParams.docBaseUrl;
+    const docBaseUrl = docParams.docBaseUrl;
-    var workerHandlerName = docParams.docId + "_worker";
+    const workerHandlerName = docParams.docId + "_worker";
-    var handler = new MessageHandler(workerHandlerName, docId, port);
+    let handler = new MessageHandler(workerHandlerName, docId, port);
 
     // Ensure that postMessage transfers are always correctly enabled/disabled,
     // to prevent "DataCloneError" in browsers without transfers support.
@@ -173,7 +172,7 @@ class WorkerMessageHandler {
 
     function finishWorkerTask(task) {
       task.finish();
-      var i = WorkerTasks.indexOf(task);
+      const i = WorkerTasks.indexOf(task);
       WorkerTasks.splice(i, 1);
     }
 
@@ -208,10 +207,10 @@ class WorkerMessageHandler {
     }
 
     function getPdfManager(data, evaluatorOptions, enableXfa) {
-      var pdfManagerCapability = createPromiseCapability();
+      const pdfManagerCapability = createPromiseCapability();
       let newPdfManager;
 
-      var source = data.source;
+      const source = data.source;
       if (source.data) {
         try {
           newPdfManager = new LocalPdfManager(
@@ -229,7 +228,7 @@ class WorkerMessageHandler {
         return pdfManagerCapability.promise;
       }
 
-      var pdfStream,
+      let pdfStream,
         cachedChunks = [];
       try {
         pdfStream = new PDFWorkerStream(handler);
@@ -238,7 +237,7 @@ class WorkerMessageHandler {
         return pdfManagerCapability.promise;
       }
 
-      var fullRequest = pdfStream.getFullReader();
+      const fullRequest = pdfStream.getFullReader();
       fullRequest.headersReady
         .then(function () {
           if (!fullRequest.isRangeSupported) {
@@ -246,7 +245,7 @@ class WorkerMessageHandler {
           }
 
           // We don't need auto-fetch when streaming is enabled.
-          var disableAutoFetch =
+          const disableAutoFetch =
             source.disableAutoFetch || fullRequest.isStreamingSupported;
           newPdfManager = new NetworkPdfManager(
             docId,
@@ -278,9 +277,9 @@ class WorkerMessageHandler {
           cancelXHRs = null;
         });
 
-      var loaded = 0;
+      let loaded = 0;
-      var flushChunks = function () {
+      const flushChunks = function () {
-        var pdfFile = arraysToBytes(cachedChunks);
+        const pdfFile = arraysToBytes(cachedChunks);
         if (source.length && pdfFile.length !== source.length) {
           warn("reported HTTP length is different from actual");
         }
@@ -300,8 +299,8 @@ class WorkerMessageHandler {
         }
         cachedChunks = [];
       };
-      var readPromise = new Promise(function (resolve, reject) {
+      const readPromise = new Promise(function (resolve, reject) {
-        var readChunk = function ({ value, done }) {
+        const readChunk = function ({ value, done }) {
           try {
             ensureNotTerminated();
             if (done) {
@@ -355,7 +354,7 @@ class WorkerMessageHandler {
         ensureNotTerminated();
 
         if (ex instanceof PasswordException) {
-          var task = new WorkerTask(`PasswordException: response ${ex.code}`);
+          const task = new WorkerTask(`PasswordException: response ${ex.code}`);
           startWorkerTask(task);
 
           handler
@@ -406,7 +405,7 @@ class WorkerMessageHandler {
 
       ensureNotTerminated();
 
-      var evaluatorOptions = {
+      const evaluatorOptions = {
         maxImageSize: data.maxImageSize,
         disableFontFace: data.disableFontFace,
         ignoreErrors: data.ignoreErrors,
@@ -656,9 +655,9 @@ class WorkerMessageHandler {
     );
 
     handler.on("GetOperatorList", function wphSetupRenderPage(data, sink) {
-      var pageIndex = data.pageIndex;
+      const pageIndex = data.pageIndex;
       pdfManager.getPage(pageIndex).then(function (page) {
-        var task = new WorkerTask(`GetOperatorList: page ${pageIndex}`);
+        const task = new WorkerTask(`GetOperatorList: page ${pageIndex}`);
         startWorkerTask(task);
 
         // NOTE: Keep this condition in sync with the `info` helper function.
@@ -707,12 +706,12 @@ class WorkerMessageHandler {
     });
 
     handler.on("GetTextContent", function wphExtractText(data, sink) {
-      var pageIndex = data.pageIndex;
+      const pageIndex = data.pageIndex;
       sink.onPull = function (desiredSize) {};
       sink.onCancel = function (reason) {};
 
       pdfManager.getPage(pageIndex).then(function (page) {
-        var task = new WorkerTask("GetTextContent: page " + pageIndex);
+        const task = new WorkerTask("GetTextContent: page " + pageIndex);
         startWorkerTask(task);
 
         // NOTE: Keep this condition in sync with the `info` helper function.
@@ -806,7 +805,7 @@ class WorkerMessageHandler {
   }
 
   static initializeFromPort(port) {
-    var handler = new MessageHandler("worker", "main", port);
+    const handler = new MessageHandler("worker", "main", port);
     WorkerMessageHandler.setup(handler, port);
     handler.send("ready", null);
   }