add gm for aa gamma

Bug: skia:
Change-Id: I021939411184a0a8a6fe3db01867ed2b6f94066b
Reviewed-on: https://skia-review.googlesource.com/c/193962
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
1 file changed