Remove the isFetchSupported function since the Fetch API is available in all supported browsers

The currently supported browsers, note the minimum versions [listed here](5a4e06af2d/gulpfile.js (L78-L88)), should now have native support for all of the features checked in the `isFetchSupported` function:

 - https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/API/Response#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/API/Body/body#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream#browser_compatibility

Hence this function can now be removed, and the code can thus be simplified a little bit.

src/display/display_utils.js

@@ -49,7 +49,7 @@ class DOMCanvasFactory extends BaseCanvasFactory {
 async function fetchData(url, asTypedArray = false) {
   if (
     (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) ||
-    (isFetchSupported() && isValidFetchUrl(url, document.baseURI))
+    isValidFetchUrl(url, document.baseURI)
   ) {
     const response = await fetch(url);
     if (!response.ok) {
@@ -483,15 +483,6 @@ class StatTimer {
   }
 }
 
-function isFetchSupported() {
-  return (
-    typeof fetch !== "undefined" &&
-    typeof Response !== "undefined" &&
-    "body" in Response.prototype &&
-    typeof ReadableStream !== "undefined"
-  );
-}
-
 function isValidFetchUrl(url, baseUrl) {
   try {
     const { protocol } = baseUrl ? new URL(url, baseUrl) : new URL(url);
@@ -626,7 +617,6 @@ export {
   getFilenameFromUrl,
   getPdfFilenameFromUrl,
   isDataScheme,
-  isFetchSupported,
   isPdfFile,
   isValidFetchUrl,
   LinkTarget,

src/pdf.js

@@ -18,7 +18,6 @@ import {
   addLinkAttributes,
   getFilenameFromUrl,
   getPdfFilenameFromUrl,
-  isFetchSupported,
   isPdfFile,
   isValidFetchUrl,
   LinkTarget,
@@ -54,6 +53,7 @@ import {
 } from "./shared/util.js";
 import { AnnotationLayer } from "./display/annotation_layer.js";
 import { GlobalWorkerOptions } from "./display/worker_options.js";
+import { isNodeJS } from "./shared/is_node.js";
 import { renderTextLayer } from "./display/text_layer.js";
 import { SVGGraphics } from "./display/svg.js";
 import { XfaLayer } from "./display/xfa_layer.js";
@@ -70,60 +70,32 @@ if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")) {
     import("pdfjs/display/network.js"),
     import("pdfjs/display/fetch_stream.js"),
   ]);
-  setPDFNetworkStreamFactory(params => {
+
-    return streamsPromise.then(streams => {
+  setPDFNetworkStreamFactory(async params => {
-      const [{ PDFNetworkStream }, { PDFFetchStream }] = streams;
+    const [{ PDFNetworkStream }, { PDFFetchStream }] = await streamsPromise;
-      if (isFetchSupported() && isValidFetchUrl(params.url)) {
+    if (isValidFetchUrl(params.url)) {
-        return new PDFFetchStream(params);
-      }
-      return new PDFNetworkStream(params);
-    });
-  });
-} else if (PDFJSDev.test("GENERIC")) {
-  const { isNodeJS } = require("./shared/is_node.js");
-  if (isNodeJS) {
-    const PDFNodeStream = require("./display/node_stream.js").PDFNodeStream;
-    setPDFNetworkStreamFactory(params => {
-      return new PDFNodeStream(params);
-    });
-  } else {
-    const PDFNetworkStream = require("./display/network.js").PDFNetworkStream;
-    let PDFFetchStream;
-    if (isFetchSupported()) {
-      PDFFetchStream = require("./display/fetch_stream.js").PDFFetchStream;
-    }
-    setPDFNetworkStreamFactory(params => {
-      if (PDFFetchStream && isValidFetchUrl(params.url)) {
-        return new PDFFetchStream(params);
-      }
-      return new PDFNetworkStream(params);
-    });
-  }
-} else if (PDFJSDev.test("CHROME")) {
-  const PDFNetworkStream = require("./display/network.js").PDFNetworkStream;
-  let PDFFetchStream;
-  const isChromeWithFetchCredentials = function () {
-    // fetch does not include credentials until Chrome 61.0.3138.0 and later.
-    // https://chromium.googlesource.com/chromium/src/+/2e231cf052ca5e68e22baf0008ac9e5e29121707
-    try {
-      // Indexed properties on window are read-only in Chrome 61.0.3151.0+
-      // https://chromium.googlesource.com/chromium/src.git/+/58ab4a971b06dec13e4edf9de8382ca6847f6190
-      window[999] = 123; // should throw. Note: JS strict mode MUST be enabled.
-      delete window[999];
-      return false;
-    } catch (e) {
-      return true;
-    }
-  };
-  if (isFetchSupported() && isChromeWithFetchCredentials()) {
-    PDFFetchStream = require("./display/fetch_stream.js").PDFFetchStream;
-  }
-  setPDFNetworkStreamFactory(params => {
-    if (PDFFetchStream && isValidFetchUrl(params.url)) {
       return new PDFFetchStream(params);
     }
     return new PDFNetworkStream(params);
   });
+} else if (PDFJSDev.test("GENERIC || CHROME")) {
+  if (PDFJSDev.test("GENERIC") && isNodeJS) {
+    const { PDFNodeStream } = require("./display/node_stream.js");
+
+    setPDFNetworkStreamFactory(params => {
+      return new PDFNodeStream(params);
+    });
+  } else {
+    const { PDFNetworkStream } = require("./display/network.js");
+    const { PDFFetchStream } = require("./display/fetch_stream.js");
+
+    setPDFNetworkStreamFactory(params => {
+      if (isValidFetchUrl(params.url)) {
+        return new PDFFetchStream(params);
+      }
+      return new PDFNetworkStream(params);
+    });
+  }
 }
 
 export {