Miscellaneous GrSurfaceProxy-related cleanup

This is pulled out of: https://skia-review.googlesource.com/c/10284/ (Remove GrSurface-derived classes from ops)

Change-Id: I083c0beefe4899b3517d0b0569bb25096809f410
Reviewed-on: https://skia-review.googlesource.com/10483
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
10 files changed