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

extensions/renderer/resources/context_menus_handlers.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.
 
 // Implementation of custom bindings for the contextMenus API.
 // This is used to implement the contextMenus API for extensions and for the
 // <webview> tag (see chrome_web_view_experimental.js).
 
 var contextMenuNatives = requireNative('context_menus');
 var sendRequest = require('sendRequest').sendRequest;
@@ skipping 120 matching lines @@
   callbacks.removeAll = createCustomCallback(function(instanceId) {
     delete contextMenus.handlers[instanceId];
   });
 
   return {
     requestHandlers: requestHandlers,
     callbacks: callbacks
   };
 }
 
-exports.create = createContextMenusHandlers;
+exports.$set('create', createContextMenusHandlers);