Stop the unmapper tasks before running a full GC. This ensures that all
freed memory is actually reusable in the following full GC. We also need
to keep freed pages around until after the GC in order to be able to
perform page flags checks on them when updating pointers. However,
when unmapper tasks are still running pages freed during the GC may be
unmapped too early.

Bug: chromium:1327132
Change-Id: I4fde7853b987975ae6ef304e89c53eb20b004d55
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3660247
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80718}