Add a note to tabs.onCreated documentation that the url might not be defined....

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 1300 matching lines @@
             "description": "Called when all the CSS has been inserted.",
             "parameters": []
           }
         ]
       }
     ],
     "events": [
       {
         "name": "onCreated",
         "type": "function",
-        "description": "Fires when a tab is created.",
+        "description": "Fires when a tab is created. Note that the tab's URL may not be set at the time this event fires, but you can listen to onUpdated events to be notified when a URL is set.",
         "parameters": [
           {
             "$ref": "Tab",
             "name": "tab",
             "description": "Details of the tab that was created."
           }
         ]
       },
       {
         "name": "onUpdated",
@@ skipping 3135 matching lines @@
                   "token": { "type": "string", "optional": true }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]