move SkMask.h to src

Bug: skia:
Change-Id: Ie51476412fb52df7cc07fdbd4dbbbfc89d9c08a7
Reviewed-on: https://skia-review.googlesource.com/104081
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed