DM: hide DM:FontMgr behind a factory

Change-Id: I4dc745479ceb1d5ca1ddb4a0904f342576e4562c
Reviewed-on: https://skia-review.googlesource.com/71240
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
4 files changed