Change HarfBuzz selectionRect rounding to match simple text version

Change the HarfBuzzShaper::selectionRect rounding logic to match that of
selectionRectForSimpleText. This ensures consistent rounding for
selection rects and avoids selection gaps due to inconsistent rounding.

BUG=366705
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/250593004

git-svn-id: svn://svn.chromium.org/blink/trunk@172546 bbb929c8-8fbe-4397-9dbb-9b2b20218538