We used to treat Heap::ReportExternalMemoryPressure just like
allocation observer marking steps. Which means that we advance
incremental marking but never finalize here immediately. This is
now problematic without a separate COMPLETE phase when we don't reach
the stack guard because we are stuck in C++ for awhile. In such cases
we might perform way more marking work than we used to.

We can fix this by finalizing marking immediately at this point when
the stack guard was already armed. Otherwise we prefer to finalize
marking in a task where we don't have a stack at all.

For this we add a new method
IncrementalMarking::AdvanceAndFinalizeIfNecessary. AdvanceFromTask
is renamed to AdvanceAndFinalizeIfComplete to make the difference
between those methods more clear.

Bug: v8:12775, chromium:1354911
Change-Id: If57bedb1a5f87923ccb8ad3fe2b60952e3843975
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3845082
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82626}