This CL fixes issues in YoungGenerationConcurrentMarkingVisitor
(YGCMV) that were discovered during an offline integration test which
uses YGCMV during MinorMC's final pause.

This also adds PopOnHold() to EmptyMarkingWorklist, in order to
process on-hold objects during final pause once concurrent marking is
working.

Bug: v8:13012

Change-Id: Ia4fef101bd974de9f5b031974cdae787dcbd3819
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3803030
Commit-Queue: Leon Bettscheider <bettscheider@google.com>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82116}