Always force GPU generated masks be top left (take 2)

Bug: 878195
Change-Id: Ie5d890597926f160743188ff1011f33e078f3d8f
Reviewed-on: https://skia-review.googlesource.com/150466
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed