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

extensions/renderer/resources/web_request_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 webRequest API.
 
 var binding = require('binding').Binding.create('webRequest');
 var sendRequest = require('sendRequest').sendRequest;
 var WebRequestEvent = require('webRequestInternal').WebRequestEvent;
 
 binding.registerCustomHook(function(api) {
   var apiFunctions = api.apiFunctions;
 
   apiFunctions.setHandleRequest('handlerBehaviorChanged', function() {
     var args = $Array.slice(arguments);
     sendRequest(this.name, args, this.definition.parameters,
                 {forIOThread: true});
   });
 });
 
 binding.registerCustomEvent(WebRequestEvent);
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());