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

extensions/renderer/resources/guest_view/guest_view.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.
 
 // This module implements a wrapper for a guestview that manages its
 // creation, attaching, and destruction.
 
 var CreateEvent = require('guestViewEvents').CreateEvent;
 var EventBindings = require('event_bindings');
 var GuestViewInternal =
@@ skipping 332 matching lines @@
   return internal.contentWindow;
 };
 
 // Returns the ID for this guestview.
 GuestView.prototype.getId = function() {
   var internal = privates(this).internal;
   return internal.id;
 };
 
 // Exports
-exports.GuestView = GuestView;
-exports.GuestViewImpl = GuestViewImpl;
-exports.ResizeEvent = ResizeEvent;
+exports.$set('GuestView', GuestView);
+exports.$set('GuestViewImpl', GuestViewImpl);
+exports.$set('ResizeEvent', ResizeEvent);