Re-enable Metal buffer suballocation with extra mutex guard.

Bug: skia:9213
Change-Id: Ia29489b00bcaaf17d4c7b6613ed4f22e87a87ffc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/224545
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed