When hitting objects that are allocated in the most recent lienar
allocation area, the concurrent marker currently has to bail out to the
main thread.

However, we only have to delay processing those objects until we are at
a safepoint, e.g. IM::Step(). With this change we flush those
on-hold-objects back to the shared queue upon performing an incremental
marking step.

Bug: chromium:694255
Change-Id: I25647d0fc581a5c4de0346bc394dc51062f65f70
Reviewed-on: https://chromium-review.googlesource.com/707315
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48424}