Add helper for idenitfying hw GrAATypes.

Change-Id: I7f4e5d87706df24ffb94e41902e0746bd149d733
Reviewed-on: https://skia-review.googlesource.com/5922
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed