Rename FontPlatformDataLinuxHarfBuzz to FontPlatformDataLinux

Rename the linux specific FontPlatformData implementation file from
FontPlatformDataLinuxHarfBuzz.cpp to FontPlatformDataLinux.cpp as all
platforms use HarfBuzz now.

R=jbroman@chromium.org

Review URL: https://codereview.chromium.org/404653004

git-svn-id: svn://svn.chromium.org/blink/trunk@178469 bbb929c8-8fbe-4397-9dbb-9b2b20218538