Use AllocationMemento::kSize instead of HeapObject::kHeaderSize

Bug: v8:12612
Change-Id: Ieae62546f10c96fe5e5bcf98f9235f0c7ef7ff77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3826248Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82431}