Use hb_direction_t in HarfBuzzShaper::HarfBuzzRun

Change HarfBuzzShaper to use hb_direction_t instead of TextDirection in
HarfBuzzRun and run cache to avoid unnecessary type conversions.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/498543003

git-svn-id: svn://svn.chromium.org/blink/trunk@180777 bbb929c8-8fbe-4397-9dbb-9b2b20218538