Black allocation is switched on at the start of incremental marking
if concurrent marking is enabled.

New space objects in the allocation area are handled by the main thread.

BUG=chromium:694255

Change-Id: I694affe11b95f51e2fe79563b2b048aaef982c03
Reviewed-on: https://chromium-review.googlesource.com/518862
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45626}