This CL tries to improve readability of TryMarkingComplete() by
splitting it up into multiple smaller methods.

It also removes StepResult::kWaitingForFinalization since this was
only used in one test which could easily be rewritten to not need this
value. This makes CombineStepResult() and Step()s return value simpler
to understand.

Bug: v8:12775
Change-Id: I981bc7b736246ab53058d1e61e3c67db0d1130b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3816668Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82319}