Change unregister tokens to be held weakly instead of strongly. This
enables the use case for an object to be used as its own unregister
token.

To avoid using an ephemeron table, FinalizationGroup's key_map is
changed to key off unregister tokens' identity hashes. Because hashes
may collide, a single key list may rarely contain multiple tokens. When
a FinalizationGroup WeakCell's token becomes unreachable, during GC, it
is removed from the the doubly linked key list and removed from the key
map if it had a unique key.

Bug: v8:8179
Change-Id: If88fd2ab196e3f9a287990ae345117a0abb2f04d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1970493
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65532}