[Extensions] More bindings hardening

chrome/renderer/resources/extensions/file_system_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 fileSystem API.
 
 var binding = require('binding').Binding.create('fileSystem');
 var sendRequest = require('sendRequest');
 
 var getFileBindingsForApi =
@@ skipping 96 matching lines @@
     $Function.apply(fileSystem.isWritableEntry, this, arguments);
   };
 
   fileSystem.chooseFile = function() {
     console.log("chrome.fileSystem.chooseFile is deprecated");
     console.log("Please use chrome.fileSystem.chooseEntry instead");
     $Function.apply(fileSystem.chooseEntry, this, arguments);
   };
 });
 
-exports.bindFileEntryCallback = bindFileEntryCallback;
-exports.binding = binding.generate();
+exports.$set('bindFileEntryCallback', bindFileEntryCallback);
+exports.$set('binding', binding.generate());