Add thread safety annotations.

Start out with spinlock. I tried to be more extensive, but some
of our abstractions confused the analysis. Will expand further
in following CLs.

Change-Id: I3e320c957d8ef427065a2b7e7d2187b7c6b0aef1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213060
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
8 files changed