[Extensions] More bindings hardening

chrome/renderer/resources/extensions/identity_custom_bindings.js

 // Copyright 2013 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 Identity API.
 
 var binding = require('binding').Binding.create('identity');
 
 binding.registerCustomHook(function(binding, id, contextType) {
   var apiFunctions = binding.apiFunctions;
   var identity = binding.compiledApi;
 
   apiFunctions.setHandleRequest('getRedirectURL', function(path) {
     if (path === null || path === undefined)
       path = '/';
     else
       path = String(path);
     if (path[0] != '/')
       path = '/' + path;
     return 'https://' + id + '.chromiumapp.org' + path;
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());