Replace some GrSurface:read/writePixels with the GrSurfaceContext equivalent

Change-Id: I7b11a323b0c74ee70f52b1bd8be376fb7188cb19
Reviewed-on: https://skia-review.googlesource.com/7204
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed