[Extensions] More bindings hardening

chrome/renderer/resources/extensions/webrtc_desktop_capture_private_custom_bindings.js

 // Copyright 2015 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 webrtcDesktopCapturePrivate API.
 
 var binding = require('binding').Binding.create('webrtcDesktopCapturePrivate');
 var sendRequest = require('sendRequest').sendRequest;
 var idGenerator = requireNative('id_generator');
 
@@ skipping 21 matching lines @@
   });
 
   apiFunctions.setHandleRequest('cancelChooseDesktopMedia', function(id) {
     if (id in pendingRequests) {
       delete pendingRequests[id];
       sendRequest(this.name, [id], this.definition.parameters, {});
     }
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());