[Extensions] More bindings hardening

chrome/renderer/resources/extensions/tab_capture_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 Tab Capture API.
 
 var binding = require('binding').Binding.create('tabCapture');
 var lastError = require('lastError');
 
 binding.registerCustomHook(function(bindingsAPI, extensionId) {
@@ skipping 42 matching lines @@
                     getErrorMessage(error, "Invalid argument(s)."),
                     request.stack,
                     function() { callback(null); });
     }
   }
 
   apiFunctions.setCustomCallback('capture', proxyToGetUserMedia);
   apiFunctions.setCustomCallback('captureOffscreenTab', proxyToGetUserMedia);
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());