[Extensions] More bindings hardening

chrome/renderer/resources/extensions/file_system_provider_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 fileSystemProvider API.
 
 var binding = require('binding').Binding.create('fileSystemProvider');
 var fileSystemProviderInternal =
     require('binding').Binding.create('fileSystemProviderInternal').generate();
 var eventBindings = require('event_bindings');
@@ skipping 329 matching lines @@
     function(args, dispatch) {
       var onSuccessCallback = function() {
         // TODO(mtomasz): To be implemented.
       };
       var onErrorCallback = function(error) {
         // TODO(mtomasz): To be implemented.
       }
       dispatch([onSuccessCallback, onErrorCallback]);
     });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());