The LocalAllocationBuffer (LAB) doesn't need to be iterable, when the
heap needs to be iterable we have explicit calls to `MakeIterable()`
anyways for the LABs.

Also creating that filler object initially isn't enough, we would need
to do this after each and every allocated object.

Change-Id: Iedb011205d7590a75ea17d518e78e340f1d4b63d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3270546
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77819}