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

extensions/renderer/resources/declarative_webrequest_custom_bindings.js

 // Copyright (c) 2012 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 declarativeWebRequest API.
 
 var binding = require('binding').Binding.create('declarativeWebRequest');
 
 var utils = require('utils');
 var validate = require('schemaUtils').validate;
@@ skipping 75 matching lines @@
     setupInstance(this, parameters, 'RemoveRequestCookie');
   };
   declarativeWebRequest.RemoveResponseCookie = function(parameters) {
     setupInstance(this, parameters, 'RemoveResponseCookie');
   };
   declarativeWebRequest.SendMessageToExtension = function(parameters) {
     setupInstance(this, parameters, 'SendMessageToExtension');
   };
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());