Add utilities to create GrSurfaceProxy & GrSurfaceContext from backend descs

Change-Id: Iff2278de8ddd6c0dff74e5cf1996702bad31217b
Reviewed-on: https://skia-review.googlesource.com/7647
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed