TableCaptionElement is not exposed to 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.
 
 // 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 56 matching lines @@
     alert,
     annotation,
     application,
     article,
     banner,
     blockquote,
     busy_indicator,
     button,
     button_drop_down,
     canvas,
+    caption,
     cell,
     check_box,
     client,
     color_well,
     column_header,
     column,
     combo_box,
     complementary,
     content_info,
     date,
@@ skipping 292 matching lines @@
     word_ends
   };
 
   [cpp_enum_prefix_override="ax"] enum AXTextDirection {
     text_direction_lr,
     text_direction_rl,
     text_direction_tb,
     text_direction_bt
   };
 };