When the heap is gracefully terminated, all the destructors are called.
CrossThreadPersistents must clear the back references (references from
CrossThreadPersistentRegion) so that further GCs on other threads will
not access freed CTPs.

To force destruction with young-gen enabled, the CL unmarks the heap on
termination.

Bug: chromium:1029379
Change-Id: I7f4a34a914ca20b50fe6d2ad493d56e0ba525ecc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3568473Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79780}