The boundary cells of the mark-bitmap can be access concurrently,
so they need to be updated with atomic CAS.

BUG=chromium:694255

Change-Id: Ibe85f00c8b4ccc61edc43b400c5b08a6d0ba620e
Reviewed-on: https://chromium-review.googlesource.com/521103
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45731}