This patch makes the check lock-free. When concurrent marking is on,
the main thread checks two marking worklist: bailout and shared.

Often the bailout worklist empty, so the emptiness check is in hot path.

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: I5c92ea3fb6c5300d653fbd27b536241851231f24
Reviewed-on: https://chromium-review.googlesource.com/602351Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47184}