Tab value for context menu onclick event should be optional.

chrome/common/extensions/api/context_menus.json

 // 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.
 
 [
   {
     "namespace": "contextMenus",
     "dependencies": [ "tabs" ],
     "types": [
       {
@@ skipping 284 matching lines @@
         "description": "Fired when a context menu item is clicked.",
         "parameters": [
           {
             "name": "info",
             "$ref": "OnClickData",
             "description": "Information about the item clicked and the context where the click happened."
           },
           {
             "name": "tab",
             "$ref": "tabs.Tab",
-            "description": "The details of the tab where the click took place."
+            "description": "The details of the tab where the click took place.",

[Mihai Parparita -not on Chrome, 2012/08/16 19:52:42]

Mention that the parameter will be missing in cases where the click did not take
place in a tab.

+            "optional": true
           }
         ]
       }
     ]
   }
 ]