Make SkString reference counting thread/TSAN safe.

BUG=skia:7107

Change-Id: I8ead98f7694faaed8e6f6f29b1fcd88501d36b66
Reviewed-on: https://skia-review.googlesource.com/54400
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed