Hide GrPixelConfig in SkSurfaceCharacterization

TBR=bsalomon@google.com
Change-Id: Ifc38d0ca2b18128d27c187f789461fea62d1edae
Reviewed-on: https://skia-review.googlesource.com/117440
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
9 files changed