[Extensions] More bindings hardening

chrome/renderer/resources/extensions/file_browser_handler_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 fileBrowserHandler API.
 
 var binding = require('binding').Binding.create('fileBrowserHandler');
 
 var eventBindings = require('event_bindings');
 var fileBrowserNatives = requireNative('file_browser_handler');
@@ skipping 36 matching lines @@
       }
 
       externalCallback(result);
     }
 
     return fileBrowserHandlerInternal.selectFile(
         selectionParams, $Function.bind(internalCallback, null, callback));
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());