[Extensions] More bindings hardening

chrome/renderer/resources/extensions/page_action_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 pageAction API.
 
 var binding = require('binding').Binding.create('pageAction');
 
 var setIcon = require('setIcon').setIcon;
 var sendRequest = require('sendRequest').sendRequest;
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setHandleRequest('setIcon', function(details, callback) {
     setIcon(details, function(args) {
       sendRequest(this.name, [args, callback], this.definition.parameters);
     }.bind(this));
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());