Add thread annotations to SkSharedMutex

Change-Id: Ieb7f4f102ee69b28f3b34c77822ff45fd2300844
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213463
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed