hr tag should return SplitterRole.

Remove redundant role HorizontalRuleRole. HR tag should return SplitterRole.

This CL depends on https://codereview.chromium.org/709523002/

BUG=384723

Review URL: https://codereview.chromium.org/703033006

git-svn-id: svn://svn.chromium.org/blink/trunk@185053 bbb929c8-8fbe-4397-9dbb-9b2b20218538