concurrent marker.

Bug: chromium:694255
Change-Id: I973ba8df7a4afc5f58ede02f3f6d043cf7038784
Reviewed-on: https://chromium-review.googlesource.com/600970Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47153}