Remove unnecessary ifdefs from platform/fonts/FontPlatformData.h

Remove in-class platform ifdefs from platform/fonts/FontPlatformData.h
as the entire file is wrapped in a OS(MACOSX) macro.

R=leviw@chromium.org, pdr@chromium.org

Review URL: https://codereview.chromium.org/471693002

git-svn-id: svn://svn.chromium.org/blink/trunk@180306 bbb929c8-8fbe-4397-9dbb-9b2b20218538