Add experimental HarfBuzz support on Windows

Add experimental HARFBUZZ_ON_WINDOWS flag toggling the use of HarfBuzz
instead of Uniscribe for complex text shaping on Windows. Also modify
the HarfBuzz code slightly to compile on Windows and add the missing
functionality to the windows font data structures to support HarfBuzz
shaping.

BUG=313423
R=eseidel@chromium.org, bungeman@chromium.org

Review URL: https://codereview.chromium.org/53083004

git-svn-id: svn://svn.chromium.org/blink/trunk@161200 bbb929c8-8fbe-4397-9dbb-9b2b20218538