Remove usePrinterFont() property of FontDescription

Since we have subpixel positioning, we can permanently switch to
using so called "printer fonts" on Mac and remove the platform
specific property from FontDescription. This permanently disables
hinting, as discussed with eseidel and behdad.
For LayoutTests, as on other platforms, we keep hinted fonts, 
only disable hinting for a group of subpixel positioning tests,
as previously done on Windows for DirectWrite testing.

BUG=373240,377696
R=eae,dglazkov,leviw

Review URL: https://codereview.chromium.org/275913005

git-svn-id: svn://svn.chromium.org/blink/trunk@174930 bbb929c8-8fbe-4397-9dbb-9b2b20218538