cppgc: Add GetWriteBarrierType that ignores slots

GetWriteBarrierType used to consider the slot so that a barrier is not
triggered for on-stack slots. For strongifying weak collections we want
the write barrier to trigger even if the backing store is only reachable
from stack.

Blink counterpart: crrev.com/c/3162170

Bug: chromium:1244057
Change-Id: I75b1ca62ad5de7bae3d2f4c1a9acce839f3ccdc1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162127


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76872}