Add onChanged callback for chrome.sessions API.

chrome/common/extensions/api/sessions.json

 // Copyright 2013 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": "sessions",
     "description": "Use the <code>chrome.sessions</code> API to query and restore tabs and windows from a browsing session.",
     "types": [
       {
@@ skipping 89 matching lines @@
               {
                 "$ref": "Session",
                 "name": "restoredSession",
                 "description": "A $(ref:sessions.Session) containing the restored $(ref:windows.Window) or $(ref:tabs.Tab) object."
               }
             ]
           }
         ]
       }
     ],
+    "events": [
+      {
+        "name": "onChanged",
+        "description": "Fired when recently closed tabs and/or windows are changed. This event does not monitor synced sessions changes.",
+        "type": "function"
+      }
+    ],
     "properties": {
       "MAX_SESSION_RESULTS": {
         "value": 25,
         "description": "The maximum number of $(ref:sessions.Session) that will be included in a requested list."
       }
     }
   }
 ]