[CRX] Disable fetch in Chrome 60-

Chrome 60 and earlier does not include credentials (cookies) in requests
made with fetch, regardless of extension permissions. This was fixed in
61.0.3138.0 by
2e231cf052

This patch disables the fetch backend in all affected Chrome versions.
The browser detection is done by checking for a change that coincides
with the release of Chrome 61.

Test case:
1. Copy the `isChromeWithFetchCredentials` function from the patch.
2. Run it in the JS console of Chrome and verify the return value.

Verified results:
- 49.0.2623.75 - false (earliest supported version by us)
- 60.0.3112.90 - false (last major version affected by bug)
- 61.0.3163.100 - true (first major version without bug)
- 65.0.3325.146 - true (current stable)

Test case 2:
1. Build the extension (`gulp chromium`) and load it in Chrome.
2. Open the developer tools, and open any PDF file.
3. In the "Network tab" of the developer tools, look at "request type".
   In Chrome 60-: Should be "xhr"
   In Chrome 61+: Should be "fetch"

src/pdf.js

@@ -52,8 +52,21 @@ if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
 } else if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('CHROME')) {
   let PDFNetworkStream = require('./display/network.js').PDFNetworkStream;
   let PDFFetchStream;
+  let 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 (typeof Response !== 'undefined' && 'body' in Response.prototype &&
-      typeof ReadableStream !== 'undefined') {
+      typeof ReadableStream !== 'undefined' && isChromeWithFetchCredentials()) {
     PDFFetchStream = require('./display/fetch_stream.js').PDFFetchStream;
   }
   pdfjsDisplayAPI.setPDFNetworkStreamFactory((params) => {