<chromium> Fix medium hinting on Linux with fontconfig and freetype

Skia freetype will enforce slight hinting in SkTypeface_FreeType::onFilterRec
when subpixel positioning is enabled. To avoid overriding medium hinting
with slight, we should not enable subpixel positioning for medium hinting
similar to how it is avoided for full hinting as well.

Change-Id: I4a229cf2b8827c11f33789d1c35c2d0eaeaed0ae
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>