Add FenceSync support to Metal

Bug: skia:8243
Change-Id: Iadd2445efe86e796b23ea20edbe49d684f626d9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212270
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed