Current spec text unclear wrt. realm

[bmeurer]

The current text as it stands doesn't make it clear in which realm the DoAgentFinalization is supposed to run, which then leaves it open which realm is used by the EnqueueJob, which in turn means that it's unclear in which realm the WeakFactoryCleanupJob has to run.

Since WeakFactoryCleanupJob creates JavaScript objects and even calls into arbitrary user code - aka [Cleanup] - which might not even be callable, it has to have some current realm.

One obvious solution here would be remember either the realm or the current execution context on the WeakFactory and enter that realm in DoAgentFinalization for that particular factory.

[littledan]

@bmeurer This suggestion sounds right to me, and mirrors similar logic in WebIDL (recently made concrete by @Ms2ger). Here, as you say, each FinalizationGroup could have its own [[Realm]] internal slot, which is used to look up the %FinalizationGroupCleanupIterator% when constructing one.