Fix GrContext::writePixels bug

Bug: 769760
Change-Id: I63603c036a8eef5eec66afb6ac4e937f556bbb63
Reviewed-on: https://skia-review.googlesource.com/59681
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed