Have the remote and main strike caches return a StrikeRef

Change-Id: Ia42ff6ecec43063dbe873a62de67c90259a09bc3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/558541
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed