[Extensions] More bindings hardening

chrome/renderer/resources/extensions/image_writer_private_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 image writer private API.
 
 var binding = require('binding').Binding.create('imageWriterPrivate');
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setUpdateArgumentsPostValidate(
       'writeFromFile', function(device, fileEntry, options, callback) {
     var fileSystemName = fileEntry.filesystem.name;
     var relativePath = $String.slice(fileEntry.fullPath, 1);
     return [device, fileSystemName, relativePath, callback];
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());