Add a mutex to SkStrike to regulate multithreaded access

This is not needed currently because the SkStrikeCache
still uses the remove/replace method of multi-threading.
This will let me see any overhead problems. The next CL
will be a multi-threaded test.

Change-Id: Ida8255cc86816fa21b4315a4bbccfbfab5ca475c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268683
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed