Fix TSAN error in skia/chromium ref count debug checks

Using std::atomic to make debug checks thread safe.

BUG=509693
R=senorblanco@chromium.org

Committed: https://crrev.com/0d030275667855c06be6cfc2ac10fb434d7ed771
Cr-Commit-Position: refs/heads/master@{#363274}

Review URL: https://codereview.chromium.org/1494633002

Cr-Commit-Position: refs/heads/master@{#366750}
2 files changed