Run AnnotationStorage.resetModified when destroying the PDFDocumentLoadingTask/PDFDocumentProxy

This will, in a very simple way using the existing events, thus allow the viewer to remove the "beforeunload" `window` event listener when the document is closed.
Generally speaking we want to avoid having *global* event listeners for the PDF document instance, which is why the `EventBus` exists, and instead reserve global events for the viewer itself. However, the `AnnotationStorage` "beforeunload" event unfortunately needs to be document-specific and we should thus ensure that it's correctly removed when the document is destroyed.

src/display/api.js

@@ -2127,6 +2127,10 @@ class WorkerTransport {
     this.setupMessageHandler();
   }
 
+  get loadingTaskSettled() {
+    return this.loadingTask._capability.settled;
+  }
+
   destroy() {
     if (this.destroyCapability) {
       return this.destroyCapability.promise;
@@ -2154,6 +2158,18 @@ class WorkerTransport {
     // We also need to wait for the worker to finish its long running tasks.
     const terminated = this.messageHandler.sendWithPromise("Terminate", null);
     waitOn.push(terminated);
+    // Allow `AnnotationStorage`-related clean-up when destroying the document.
+    if (this.loadingTaskSettled) {
+      const annotationStorageResetModified = this.loadingTask.promise
+        .then(pdfDocument => {
+          // Avoid initializing the `annotationStorage` if it doesn't exist.
+          if (pdfDocument.hasOwnProperty("annotationStorage")) {
+            pdfDocument.annotationStorage.resetModified();
+          }
+        })
+        .catch(() => {});
+      waitOn.push(annotationStorageResetModified);
+    }
     Promise.all(waitOn).then(() => {
       this.commonObjs.clear();
       this.fontLoader.clear();