The main fix is to ensure that the recently allocated object is marked
black in StressMarkingObserver::Step. Otherwise, the concurrent marker
can observe an uninitialized white object in the old generation.

This patch also removes the --black-allocation flag.

Bug: v8:8676
Change-Id: Iba8f00330eabc4847eaef2cd3dfb2884d62a48b4
Reviewed-on: https://chromium-review.googlesource.com/c/1425915
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59002}