AX_ROLE_HORIZONTAL_RULE is not required.

content/renderer/accessibility/blink_ax_enum_conversion.cc

 // Copyright (c) 2013 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.
 
 #include "content/renderer/accessibility/blink_ax_enum_conversion.h"
 
 #include "base/logging.h"
 
 namespace content {
 
@@ skipping 152 matching lines @@
     case blink::WebAXRoleGrid:
       return ui::AX_ROLE_GRID;
     case blink::WebAXRoleGroup:
       return ui::AX_ROLE_GROUP;
     case blink::WebAXRoleGrowArea:
       return ui::AX_ROLE_GROW_AREA;
     case blink::WebAXRoleHeading:
       return ui::AX_ROLE_HEADING;
     case blink::WebAXRoleHelpTag:
       return ui::AX_ROLE_HELP_TAG;
-    case blink::WebAXRoleHorizontalRule:
-      return ui::AX_ROLE_HORIZONTAL_RULE;
     case blink::WebAXRoleIframe:
       return ui::AX_ROLE_IFRAME;
     case blink::WebAXRoleIgnored:
       return ui::AX_ROLE_IGNORED;
     case blink::WebAXRoleImage:
       return ui::AX_ROLE_IMAGE;
     case blink::WebAXRoleImageMap:
       return ui::AX_ROLE_IMAGE_MAP;
     case blink::WebAXRoleImageMapLink:
       return ui::AX_ROLE_IMAGE_MAP_LINK;
@@ skipping 225 matching lines @@
     case blink::WebAXTextDirectionBT:
       return ui::AX_TEXT_DIRECTION_BT;
     default:
       NOTREACHED();
   }
 
   return ui::AX_TEXT_DIRECTION_NONE;
 }
 
 }  // namespace content