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

extensions/renderer/resources/guest_view/extension_view/extension_view.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.
 
 // This module implements the ExtensionView <extensionview>.
 
 var GuestViewContainer = require('guestViewContainer').GuestViewContainer;
 var ExtensionViewConstants =
     require('extensionViewConstants').ExtensionViewConstants;
 var ExtensionViewEvents = require('extensionViewEvents').ExtensionViewEvents;
@@ skipping 118 matching lines @@
           }
         }.bind(this));
       }
     }.bind(this));
   }
 };
 
 GuestViewContainer.registerElement(ExtensionViewImpl);
 
 // Exports.
-exports.ExtensionViewImpl = ExtensionViewImpl;
+exports.$set('ExtensionViewImpl', ExtensionViewImpl);