Merge FontPlatformDataHarfBuzz and FontPlatformData headers

Those two files were split apart and redundantly copied
for non-Mac and Mac platforms. This is an attempt an unifying
them, as a preparation for removing the CoreText shaper.
We can merge the implementations in FontPlatformDataHarfBuzz.cpp
and FontPlatformData.cpp as a next step.

This is a reland after the change got reverted again
in 7443ae0830de1c3483 due to Windows build breakge.

BUG=334269, 411287
R=eae,arv

Review URL: https://codereview.chromium.org/559433002

git-svn-id: svn://svn.chromium.org/blink/trunk@181655 bbb929c8-8fbe-4397-9dbb-9b2b20218538