This makes popping from the marking deque safe for concurrent marking.

BUG=chromium:694255

Change-Id: I3edf8ece3d3c3dd8f045b3ea2f8196b322a56a54
Reviewed-on: https://chromium-review.googlesource.com/527154
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45806}