Make HTML5 <mark> accessible.

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.
 
 // 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 three enums, also add to
   // chrome/common/extensions/api/automation.idl.
@@ skipping 107 matching lines @@
     line_break,
     link,
     list_box_option,
     list_box,
     list_item,
     list_marker,
     list,
     location_bar,
     log,
     main,
+    mark,
     marquee,
     math,
     menu_bar,
     menu_button,
     menu_item,
     menu_item_check_box,
     menu_item_radio,
     menu_list_option,
     menu_list_popup,
     menu,
@@ skipping 308 matching lines @@
   };
 
   [cpp_enum_prefix_override="ax"] enum AXSortDirection {
     sort_direction_unsorted,
     sort_direction_ascending,
     sort_direction_descending,
     sort_direction_other
   };
 
 };