[Extensions] More bindings hardening

chrome/renderer/resources/extensions/developer_private_custom_bindings.js

 // Copyright 2014 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 developerPrivate API.
 
 var binding = require('binding').Binding.create('developerPrivate');
 
 binding.registerCustomHook(function(bindingsAPI) {
   var apiFunctions = bindingsAPI.apiFunctions;
@@ skipping 42 matching lines @@
     }
     chrome.developerPrivate.openDevTools({
         extensionId: options.extension_id,
         renderProcessId: renderProcessId,
         renderViewId: renderViewId,
         incognito: options.incognito
     }, callback);
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());