heap: Add flushing of not fully constructed objects

Upon entering an incremental step without stack such objects can be
flushed to a worklist that is processed in the regular marking loop as
the objects are definitely constructed at this point.

Bug: 843903
Change-Id: Ie4acbb7a58e7bedaee3b7806bdd18ca306ff14d7
Reviewed-on: https://chromium-review.googlesource.com/c/1350751
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612780}
8 files changed