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

extensions/renderer/resources/guest_view/guest_view_container.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 the shared functionality for different guestview
 // containers, such as web_view, app_view, etc.
 
 var DocumentNatives = requireNative('document_natives');
 var GuestView = require('guestView').GuestView;
 var GuestViewInternalNatives = requireNative('guest_view_internal');
@@ skipping 288 matching lines @@
 
   // Delete the callbacks so developers cannot call them and produce unexpected
   // behavior.
   delete proto.createdCallback;
   delete proto.attachedCallback;
   delete proto.detachedCallback;
   delete proto.attributeChangedCallback;
 }
 
 // Exports.
-exports.GuestViewContainer = GuestViewContainer;
+exports.$set('GuestViewContainer', GuestViewContainer);