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

extensions/renderer/resources/permissions_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 Permissions API.
 
 var binding = require('binding').Binding.create('permissions');
 
 var Event = require('event_bindings').Event;
 
@@ skipping 71 matching lines @@
   permissions.onAdded.dispatchToListener = function(callback, args) {
     for (var i = 0; i < args[0].permissions.length; i += 1) {
       args[0].permissions[i] = maybeConvertToObject(args[0].permissions[i]);
     }
     $Function.call(Event.prototype.dispatchToListener, this, callback, args);
   };
   permissions.onRemoved.dispatchToListener =
       permissions.onAdded.dispatchToListener;
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());