Made notification center notifications collapsed and expandable.

ui/message_center/message_center.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 22 matching lines @@
         'message_center_export.h',
         'message_center_switches.cc',
         'message_center_switches.h',
         'message_center_tray.cc',
         'message_center_tray.h',
         'message_center_tray_delegate.h',
         'message_center_util.cc',
         'message_center_util.h',
         'notification.cc',
         'notification.h',
+        'notification_change_observer.h',
         'notification_list.cc',
         'notification_list.h',
         'notification_types.cc',
         'notification_types.h',
         'notifier_settings_view_delegate.cc',
         'notifier_settings_view_delegate.h',
         'views/message_bubble_base.cc',
         'views/message_bubble_base.h',
         'views/message_center_bubble.cc',
         'views/message_center_bubble.h',
@@ skipping 25 matching lines @@
         '../ui.gyp:ui',
         'message_center',
       ],
       'sources': [
         'message_center_tray_unittest.cc',
         'notification_list_unittest.cc',
       ],
     },
   ],
 }