Share code between FontPlatformDataWin and FontPlatformDataHarfBuzz

Move linux specific code from FontPlatformDataHarfBuzz.cpp into a new
FontPlatformDataLinux.cpp file and merge the shared code across the win
and linux implementations into FontPlatformDataHarfBuzz.cpp.

With this change code shared across all HarfBuzz platforms is in
FontPlatformDataHarfBuzz.cpp and platform specific code in the Linux and
Windoows files respectively.

Once Mac is fully moved over to HarfBuzz FontPlatformDataHarfBuzz will
be renamed to FontPlatformData and moved to platform/fonts.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/214383007

git-svn-id: svn://svn.chromium.org/blink/trunk@170801 bbb929c8-8fbe-4397-9dbb-9b2b20218538