A WeakCell's unregister_token and holdings are passed in by the user,
and so should be JSAny. A FinalizationRegistry's callback function must
always be Callable.

Bug: v8:8179
Change-Id: I8e447ce4bfeba712594fc9a7c9923ce718adcd62
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2129273Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66996}