[Extensions] More bindings hardening

chrome/renderer/resources/extensions/media_galleries_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 Media Gallery API.
 
 var binding = require('binding').Binding.create('mediaGalleries');
 var blobNatives = requireNative('blob_natives');
 var mediaGalleriesNatives = requireNative('mediaGalleries');
 
@@ skipping 84 matching lines @@
     if (callback)
       callback(response ? response.metadata : null);
 
     // The UUID was in position 0 in the setUpdateArgumentsPostValidate
     // function.
     var uuid = request.args[0];
     delete blobsAwaitingMetadata[uuid];
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());