We need to check whether ephemeron key is fully constructed to sync the
mark bit before checking it.

Bug: chromium:1246730
Change-Id: I3ba69898202c1df94833a0bc7442b2be0e61694e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3143993Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76678}