Replace non-null pointer in ObjectAllocator with references.

Bug: chromium:1056170
Change-Id: I55124610490bb903819f88a70b1f8e0fea4e430d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3132969
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76607}