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 original change 72f392cc147764ec75e4f
got reverted in bcfdb1312a26b0e611. Fix is to
change to the previous operator== implementation and add
m_isHashTableDeletedValue to the hash computation. 

I believe this addresses the observed crash issue as well.

BUG=334269, 411287
R=eae,arv

Review URL: https://codereview.chromium.org/542653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181541 bbb929c8-8fbe-4397-9dbb-9b2b20218538