Add asStrikeForGPU to StrikeRef

Change-Id: I5596be24e1e4a3f64659b739c672daa2b6170368
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/558654
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed