[Extensions] More bindings hardening

chrome/renderer/resources/extensions/webstore_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 webstore API.
 
 var webstoreNatives = requireNative('webstore');
 var Event = require('event_bindings').Event;
 
 function Installer() {
@@ skipping 67 matching lines @@
 var installer = new Installer();
 
 var chromeWebstore = {
   install: function (url, onSuccess, onFailure) {
     installer.install(url, onSuccess, onFailure);
   },
   onInstallStageChanged: installer.onInstallStageChanged,
   onDownloadProgress: installer.onDownloadProgress
 };
 
-exports.binding = chromeWebstore;
+exports.$set('binding', chromeWebstore);
 
 // Called by webstore_bindings.cc.
 exports.onInstallResponse =
     Installer.prototype.onInstallResponse.bind(installer);
 exports.onInstallStageChanged =
     Installer.prototype.onInstallStageChanged.bind(installer);
 exports.onDownloadProgress =
     Installer.prototype.onDownloadProgress.bind(installer);