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

extensions/renderer/resources/guest_view/extension_options/extension_options_events.js

 // Copyright 2014 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 CreateEvent = require('guestViewEvents').CreateEvent;
 var GuestViewEvents = require('guestViewEvents').GuestViewEvents;
 
 function ExtensionOptionsEvents(extensionOptionsImpl) {
   GuestViewEvents.call(this, extensionOptionsImpl);
 
@@ skipping 19 matching lines @@
     evt: CreateEvent('extensionOptionsInternal.onPreferredSizeChanged'),
     fields:['width', 'height']
   }
 }
 
 ExtensionOptionsEvents.prototype.getEvents = function() {
   return ExtensionOptionsEvents.EVENTS;
 };
 
 // Exports.
-exports.ExtensionOptionsEvents = ExtensionOptionsEvents;
+exports.$set('ExtensionOptionsEvents', ExtensionOptionsEvents);