Introducing IncrementalMarking::AdvanceForTesting as last bottleneck
for driving incremental marking in addition to AdvanceFromTask
and AdvanceOnAllocation.

Now that we have those 3 bottlenecks, Step() and AdvanceWithDeadline()
can become private methods in IncrementalMarking. We also don't need
the StepResult return value in Step() anymore, which allows us to
remove CombineStepResult.

Bug: v8:12775
Change-Id: I702714439ef7ea4b9abf2156387503d4d00a7a48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3823131Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82552}