Add GrSurfaceProxy* parameter to GrContext::flush

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

Change-Id: I12c66a6fa826c2363b21ece56391fb352d12d6b3
Reviewed-on: https://skia-review.googlesource.com/10541
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed