[Extensions] More bindings hardening

chrome/renderer/resources/extensions/tts_engine_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 ttsEngine API.
 
 var binding = require('binding').Binding.create('ttsEngine');
 
 var eventBindings = require('event_bindings');
 
 eventBindings.registerArgumentMassager('ttsEngine.onSpeak',
     function(args, dispatch) {
   var text = args[0];
   var options = args[1];
   var requestId = args[2];
   var sendTtsEvent = function(event) {
     chrome.ttsEngine.sendTtsEvent(requestId, event);
   };
   dispatch([text, options, sendTtsEvent]);
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());