[Extensions] Don't allow built-in extensions code to be overridden

extensions/renderer/resources/guest_view/extension_view/extension_view_events.js

 // Copyright 2015 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.
 
 // Event management for ExtensionView.
 
 var CreateEvent = require('guestViewEvents').CreateEvent;
 var GuestViewEvents = require('guestViewEvents').GuestViewEvents;
 
 function ExtensionViewEvents(extensionViewImpl) {
@@ skipping 11 matching lines @@
 };
 
 ExtensionViewEvents.prototype.getEvents = function() {
   return ExtensionViewEvents.EVENTS;
 };
 
 ExtensionViewEvents.prototype.handleLoadCommitEvent = function(event) {
   this.view.onLoadCommit(event.url);
 };
 
-exports.ExtensionViewEvents = ExtensionViewEvents;
+exports.$set('ExtensionViewEvents', ExtensionViewEvents);