Re-do close watcher user activation tracking

As noted in whatwg#10046, the close watcher anti-abuse measures were overly strict, and would prevent firing the cancel event even when the close watcher was created with user interaction.

Additionally, the "is grouped with previous" infrastructure used to track close watcher groups was buggy, since close watchers could be destroyed and this would cause groups to spuriously collapse.

To fix both of these problems, we re-do the close watcher anti-abuse protections. We now track the groups as a list-of-lists, and explicitly track how many groups should be allowed at a given time. We can then compare them when making decisions about whether to group a new close watcher, or whether to fire a cancel event.

Note that the initial fix proposed in whatwg#10046 (and implemented in whatwg#10048) is incorrect, as it allows indefinite trapping of the user.

Closes whatwg#10046.