...when handing out validity cells to handlers; because invalidating said cells is
the only time we'll need the user registrations.
Along the way, fix a corner case in WeakFixedArray, which can now be empty after
the recently introduced compaction support.

This reverts commit 968715c6.

Original review: https://codereview.chromium.org/1104813004/

Review URL: https://codereview.chromium.org/1110513002

Cr-Commit-Position: refs/heads/master@{#28076}