cppgc: Add WasConservativeGC to HeapState

This CL adds WasConservativeGC to HeapState which reports whether the
last GC was finalized conservatively. The state is updated at the end of
marking atomic pause.

Currently the library integration in Blink ignores the stack state when
scheduling a forced GC for testing. That means that we always schedule
another GC after a forced GC.
This causes a crash in web_tests which assume no GC is happening
between forced GCs if the thread is not idle and no new allocations
happen.

Usage CL: https://crrev.com/c/2720201

Drive by: Fix stack state for MarkingVerifier in CppHeap.

Bug: chromium:1056170
Change-Id: I6ad23ed7c1a53fae96425b968bc4b3eb18ce80b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2720279


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73064}