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

extensions/renderer/resources/binding.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.
 
 var Event = require('event_bindings').Event;
 var forEach = require('utils').forEach;
 var GetAvailability = requireNative('v8_context').GetAvailability;
 var exceptionHandler = require('uncaught_exception_handler');
 var lastError = require('lastError');
 var logActivity = requireNative('activityLogger');
@@ skipping 506 matching lines @@
       console.error('chrome.' + schema.namespace + ' is not available: ' +
                     availability.message);
       return;
     }
 
     this.runHooks_(mod);
     return mod;
   }
 };
 
-exports.Binding = Binding;
+exports.$set('Binding', Binding);