Background threads can now start incremental marking when necessary. In
contrast to the main thread they always need to schedule a job and can't
start incremental marking right away. Background threads also use a
simpler heuristic for deciding whether to start incremental marking.

Bug: v8:10315
Change-Id: I2b94e8273c8be860157fe9670797048ed1c5c3da
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184149Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67675}