Add an accessibility description for notification

ui/accessibility/ax_enums.idl

 // Copyright 2014 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.
 
 // TODO(nektar): Migrate entire file to Mojoq.
 // These should be kept in sync with third_party/WebKit/public/web/WebAXEnums.h
 // until the Chromium and Blink trees are merged.
 [camel_case_enum_to_string=true] namespace ui {
 
   // For new entries to the following four enums, also add to
@@ skipping 141 matching lines @@
     menu_bar,
     menu_button,
     menu_item,
     menu_item_check_box,
     menu_item_radio,
     menu_list_option,
     menu_list_popup,
     meter,
     navigation,
     note,
+    notification,
     outline,
     pane,
     paragraph,
     pop_up_button,
     pre,
     presentational,
     progress_indicator,
     radio_button,
     radio_group,
     region,
@@ skipping 490 matching lines @@
     // First node is before the second one.
     before,
 
     // Nodes are the same.
     equal,
 
     // First node is after the second one.
     after
   };
 };