IsMarking returns true as long as a marker exists. That means IsMarking
is true during weak processing as well.
ActiveScriptWrappableManager in blink uses a weak callback that updates
a HeapVector and thus can trigger a write barrier during the atomic
pause (which violates a DCHECK in the barrier).

Bug: chromium:1056170
Change-Id: I6304b38da9751320836a5e2407e8c7d529367bad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2700676Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72831}