When should DoAgentFinalization be called?

[gsathya]

Is this something that should be embedder defined?

cc @dtribble @erights @marjakh

[tschneidereit]

The timing of DoAgentFinalization should probably be wholly implementation defined. In practice, I'd expect it to happen at the end of the sweep phase, but for engines with incremental or concurrent sweeping, there's no fundamental reason why it couldn't happen after each slice/in the process of the concurrent sweeping.

The results of this become visible through cleanupSome, which should probably always invoke the callback, and whose iterator should yield items as long as the queue isn't empty.

Another question is when this queue should be processed automatically, invoking the cleanup callback passed to the WeakFactory's constructor. On that, I think we should specify that it must not happen before the end of the current task, but that otherwise the timing is implementation defined, too.

[marjakh]

DoAgentFinalization seems to assume to have a current Realm ("native context") though, and during GC there's none. See also #38 for a suggested fix.

[littledan]

We ended up making this call embedder-defined. See #86 for details.