[TextBlob] Plumb a Font param for setupPaint().

Similarly to the non-blob code path, we should pass a Font arg to
setupPaint() for Mac hinting heuristics.

(See https://codereview.chromium.org/692643006)

BUG=421412
R=eae@chromium.org,jbroman@chromium.org,bungeman@chromium.org

Review URL: https://codereview.chromium.org/692643007

git-svn-id: svn://svn.chromium.org/blink/trunk@185082 bbb929c8-8fbe-4397-9dbb-9b2b20218538