make SkDestinationSurfaceColorMode private

R=brianosman@google.com, reed@google.com
Bug: skia:
Change-Id: Ibbf91a664cc504737137717d9170ce4de09b017c
Reviewed-on: https://skia-review.googlesource.com/111161
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>
5 files changed