Background tasks read this flag, which creates a data race. This patch
works around the data races by making the access to the flag atomic.

The actual fix will be to not mutate the flag.

Bug: chromium:794911
Change-Id: Idcf03b7a1037e876036918418ce989b420784428
Reviewed-on: https://chromium-review.googlesource.com/834508Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50215}