Make HarfBuzzShaper compile on Windows

Fix a couple of type conversion issues in HarfBuzzShaper allowing it
to be compiled on Windows.

BUG=313423
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/49873002

git-svn-id: svn://svn.chromium.org/blink/trunk@161203 bbb929c8-8fbe-4397-9dbb-9b2b20218538