We have a heuristics that allows the main thread to skip marking work
by the amount marked by the background tasks.
Scavenges that happen during incremental marking can reclaim marked
objects, which should be accounted in the main thread marking schedule.

Bug: chromium:789530
Change-Id: I9b922e05202e3b7665ec191c4fe303dbe7f72a50
Reviewed-on: https://chromium-review.googlesource.com/840843
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50285}