[Extensions] More bindings hardening

chrome/renderer/resources/extensions/gcm_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 GCM API.
 
 var binding = require('binding').Binding.create('gcm');
 var forEach = require('utils').forEach;
 
 binding.registerCustomHook(function(bindingsAPI) {
@@ skipping 23 matching lines @@
         throw new Error("Payload exceeded allowed size limit. Payload size is: "
             + payloadSize);
 
       if (payloadSize == 0)
         throw new Error("No data to send.");
 
       return arguments;
     });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());