The unregister_token slot is iterated as a custom weak pointer slot,
which means the heap verifier treats it as a strong slot. Currently,
popped WeakCells (that is, WeakCells for which the owning
FinalizationRegistry's finalizer has already been invoked) neither
clears out the unregister_token slot nor marks it, which trips the heap
verifier.

Bug: chromium:1102161
Change-Id: I0a803f12379fc9df6935bc8331b3d5ecb199571a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284202
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68723}