Tab value for context menu onclick event should be optional.

chrome/test/data/extensions/platform_apps/context_menu_click/test.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.
 
 chrome.app.runtime.onLaunched.addListener(function() {
   chrome.contextMenus.create({
         id: 'id1',
         title: 'Extension Item 1',
       },
       function() {
         chrome.app.window.create('main.html', {}, function() {});
       });
 });
+
+chrome.contextMenus.onClicked.addListener(function(info, tab) {
+  chrome.test.assertEq(undefined, tab);
+  chrome.test.sendMessage("onClicked fired for " + info.menuItemId);
+});