Restore webkit-font-smoothing.

With https://codereview.chromium.org/175253002 (Blink r182920)
webkit-font-smoothing was broken on Mac. This restores
the previous behavior.

BUG=chromium:421412

Review URL: https://codereview.chromium.org/692643006

git-svn-id: svn://svn.chromium.org/blink/trunk@185065 bbb929c8-8fbe-4397-9dbb-9b2b20218538