(The bug didn't affect any functionality; we just left detached WeakCells in
inconsistent state.)

BUG=v8:8179

Change-Id: I28f6c27532383b94bdfd746db903096f1dc6f1cc
Reviewed-on: https://chromium-review.googlesource.com/c/1447651Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59255}