With concurrent marking the write barrier should trigger even if the
object is black because the concurrent marker could have fetched
object field before marking the object black.

BUG=chromium:694255

Change-Id: Icacc5672defeec85936e37d7d06780c74b97732c
Reviewed-on: https://chromium-review.googlesource.com/533614Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45938}