AX_ROLE_HORIZONTAL_RULE is not required.

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 88 matching lines @@
     embedded_object,
     figcaption,
     figure,
     footer,
     form,
     grid,
     group,
     grow_area,
     heading,
     help_tag,
-    horizontal_rule,
     iframe,
     ignored,
     image_map_link,
     image_map,
     image,
     incrementor,
     inline_text_box,
     label_text,
     legend,
     link,
@@ skipping 265 matching lines @@
     word_ends
   };
 
   [cpp_enum_prefix_override="ax"] enum AXTextDirection {
     text_direction_lr,
     text_direction_rl,
     text_direction_tb,
     text_direction_bt
   };
 };