Exposing hr tag with SplitterRole.

This is the third patch in series to remove the redundant role HorizontalRule Role.

This CL depends on https://codereview.chromium.org/703033006/

BUG=384723

Review URL: https://codereview.chromium.org/685783005

Cr-Commit-Position: refs/heads/master@{#303616}
1 file changed