[M54 merge] Lock down creation of blob:chrome-extension URLs from non-extension processes.

chrome/test/data/extensions/platform_apps/web_view/guest_focus_test/guest.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var LOG = function(msg) {
   window.console.log(msg);
 };
 
 var embedder;
 
 function sendMessageToEmbedder(message) {
   if (!embedder) {
     LOG('no embedder channel to send postMessage');
     return;
   }
   embedder.postMessage(JSON.stringify([message]), '*');
 }
 
 window.addEventListener('message', function(e) {
+  URL.createObjectURL(new Blob([]));
   embedder = e.source;
   var data = JSON.parse(e.data);
   if (data[0] == 'connect') {
     sendMessageToEmbedder('connected');
   }
 });