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

extensions/renderer/resources/guest_view/extension_options/extension_options.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 ExtensionOptionsConstants =
     require('extensionOptionsConstants').ExtensionOptionsConstants;
 var ExtensionOptionsEvents =
     require('extensionOptionsEvents').ExtensionOptionsEvents;
 var GuestViewContainer = require('guestViewContainer').GuestViewContainer;
 
@@ skipping 31 matching lines @@
       this.dispatchEvent(createFailedEvent);
     } else {
       this.attachWindow$();
     }
   }.bind(this));
 };
 
 GuestViewContainer.registerElement(ExtensionOptionsImpl);
 
 // Exports.
-exports.ExtensionOptionsImpl = ExtensionOptionsImpl;
+exports.$set('ExtensionOptionsImpl', ExtensionOptionsImpl);