Make global use-analytic-AA bit threadsafe.

I also had to cut it down to just a global atomic bool... as a field in a global singleton accessed through instance(), it's very hard to make threadsafe.

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-TSAN-Trybot

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2937

Change-Id: If80be987906dd521fbe644d1d0d577009f06d0e3
Reviewed-on: https://skia-review.googlesource.com/2937
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
7 files changed