writePixels should notify GPU that pixels have changed

R=reed@google.com,robertphillips@google.com
Bug: skia:7644
Change-Id: I469c537a649e4d8d05a78cedb26caa3057d824c1
Reviewed-on: https://skia-review.googlesource.com/108961
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>
2 files changed