Update SkBufferHead fRefCnt to std::atomic.

This fixes the noisiest bit of a debug tsan build.

Change-Id: I5df8cb3003c73640c04bd73e0aad1aefd87f833e
Reviewed-on: https://skia-review.googlesource.com/55480
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed