Since we are reading an Object field, it could be that the gc
predicate fails. Therefore, this CL changes to TryMakeRef, and
makes the return value of length_unsafe() optional.

Bug: v8:7790, v8:12282
Change-Id: I86a8bcc6649d5e8121e52f8947b8331fcf242887
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3200078Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77209}