Move SkFontMgr and SkFontStyle impls to own cpp.

SkFontMgr and SkFontStyle implementations are currently burried in the
old SkFontHost.cpp file. Move these implementations to their own file
so that the implementations are easier to find, and to make clearer that
SkFontHost.cpp needs to be removed.

Review URL: https://codereview.chromium.org/799533004
4 files changed