Added a class acting as a fake caret for accessibility.

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 75 matching lines @@
     article,
     audio,
     banner,
     blockquote,
     // TODO(nektar): Remove busy_indicator because it's used nowhere.
     busy_indicator,
     button,
     button_drop_down,  // Not used on Web.
     canvas,
     caption,
+    caret,
     cell,
     check_box,
     client,
     color_well,
     column_header,
     column,
     combo_box,
     complementary,
     content_info,
     date,
@@ skipping 556 matching lines @@
     // First node is before the second one.
     before,
 
     // Nodes are the same.
     equal,
 
     // First node is after the second one.
     after
   };
 };