For the standalone library, some platform implementations might not
support non-nested tasks. We can still offer incremental marking in
such cases using regular tasks and without assuming an empty stack.
(cppgc's default platform e.g. doesn't support non-nested tasks.)

This CL also updates GCInvoker to not trigger an incremental GC if we
won't be able to finalize it. That makes finalizing through an
non-nested incremental task safe.

Bug: chromium:1056170
Change-Id: I85f0c9f2efe643cb87dd65d80417eea0d6ee5d52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2414217
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69971}