[Extensions] More bindings hardening

chrome/renderer/resources/extensions/cast_streaming_udp_transport_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 Cast Streaming UdpTransport API.
 
 var binding = require('binding').Binding.create('cast.streaming.udpTransport');
 var natives = requireNative('cast_streaming_natives');
 
 binding.registerCustomHook(function(bindingsAPI, extensionId) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setHandleRequest('destroy', function(transportId) {
     natives.DestroyCastUdpTransport(transportId);
   });
   apiFunctions.setHandleRequest('setDestination',
     function(transportId, destination) {
       natives.SetDestinationCastUdpTransport(transportId, destination);
   });
   apiFunctions.setHandleRequest('setOptions',
     function(transportId, options) {
       natives.SetOptionsCastUdpTransport(transportId, options);
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());