The 6.3 branch has a data race that is fixed in 6.4 but the fix is too
large for back merging.

This CL will be back-merged to 6.3 after getting Canary coverage.

Concurrent marking will be re-enabled afterwards.

Bug: chromium:774644
Change-Id: I4112da0e133a637cc4fb52dee2e4c165cdc74f1f
Reviewed-on: https://chromium-review.googlesource.com/749811Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49080}