[Extensions] More bindings hardening

chrome/renderer/resources/extensions/log_private_custom_bindings.js

 // Copyright 2014 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 logPrivate API.
 var binding = require('binding').Binding.create('logPrivate');
 var sendRequest = require('sendRequest');
 
 var getFileBindingsForApi =
     require('fileEntryBindingUtil').getFileBindingsForApi;
 var fileBindings = getFileBindingsForApi('logPrivate');
 var bindFileEntryCallback = fileBindings.bindFileEntryCallback;
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
   var fileSystem = bindingsAPI.compiledApi;
 
   $Array.forEach(['dumpLogs'],
                   function(functionName) {
     bindFileEntryCallback(functionName, apiFunctions);
   });
 
 });
 
-exports.bindFileEntryCallback = bindFileEntryCallback;
-exports.binding = binding.generate();
+exports.$set('bindFileEntryCallback', bindFileEntryCallback);
+exports.$set('binding', binding.generate());