hr tag should return SplitterRole.

Source/core/accessibility/AXObject.h

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 77 matching lines @@
     EmbeddedObjectRole,
     FigcaptionRole,
     FigureRole,
     FooterRole,
     FormRole,
     GridRole,
     GroupRole,
     GrowAreaRole,
     HeadingRole,
     HelpTagRole,
-    HorizontalRuleRole,
     IframeRole,
     IgnoredRole,
     ImageMapLinkRole,
     ImageMapRole,
     ImageRole,
     IncrementorRole,
     InlineTextBoxRole,
     LabelRole,
     LegendRole,
     LinkRole,
@@ skipping 480 matching lines @@
 
     void updateCachedAttributeValuesIfNeeded() const;
 };
 
 #define DEFINE_AX_OBJECT_TYPE_CASTS(thisType, predicate) \
     DEFINE_TYPE_CASTS(thisType, AXObject, object, object->predicate, object.predicate)
 
 } // namespace blink
 
 #endif // AXObject_h