Switch wtf/SpinLock to std::atomic

Tsan identified a racy read in the existing spin lock loop.
Tsan is correct in the absolute sense, even though the read is
safe in practice. However, the architectures we support might
change, or the behavior of the compiler might change, so the
the best fix is to just use the proper std::atomic type,
because that is guaranteed to always be safe. And the compiled
code should be effectively unchanged.

BUG=556469
NOTRY=True

Committed: https://crrev.com/5e58730d6fe40ef7cead75954ef5b1c05dea6d4b
Cr-Commit-Position: refs/heads/master@{#363347}

Review URL: https://codereview.chromium.org/1463683002

Cr-Commit-Position: refs/heads/master@{#363353}
10 files changed