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

extensions/renderer/resources/storage_area.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 normalizeArgumentsAndValidate =
     require('schemaUtils').normalizeArgumentsAndValidate
 var sendRequest = require('sendRequest').sendRequest;
 
 function extendSchema(schema) {
   var extendedSchema = $Array.slice(schema);
@@ skipping 19 matching lines @@
           'storage.' + functionName,
           $Array.concat([namespace], args),
           extendSchema(funSchema.definition.parameters),
           {preserveNullInObjects: true});
     };
   }
   var apiFunctions = ['get', 'set', 'remove', 'clear', 'getBytesInUse'];
   $Array.forEach(apiFunctions, bindApiFunction);
 }
 
-exports.StorageArea = StorageArea;
+exports.$set('StorageArea', StorageArea);