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

extensions/renderer/resources/context_menus_custom_bindings.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.
 
 // Custom binding for the contextMenus API.
 
 var binding = require('binding').Binding.create('contextMenus');
 var contextMenusHandlers = require('contextMenusHandlers');
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   var handlers = contextMenusHandlers.create(false /* isWebview */);
 
   apiFunctions.setHandleRequest('create', handlers.requestHandlers.create);
 
   apiFunctions.setCustomCallback('create', handlers.callbacks.create);
 
   apiFunctions.setCustomCallback('remove', handlers.callbacks.remove);
 
   apiFunctions.setCustomCallback('update', handlers.callbacks.update);
 
   apiFunctions.setCustomCallback('removeAll', handlers.callbacks.removeAll);
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());