We need to prevent these mutation observers from being garbage
collected, and since they are only part of SimilarOriginWindowAgent and
themselves as part of the intrusive list, nobody is visiting them.
Make the list of pending mutation observers a GC::RootVector so we keep
them alive until they have been processed in the microtask.
Restores 1400+ WPT subtest passes in `dom/nodes/Element-classlist.html`.
In the current spec, MutationObservers are explicitly added to the
pending mutation observers list, and they are removed when that list is
cleared in the "notify mutation observers" microtask.
This solves some issues with slotchange events.
As noted, we delay actually emptying the list of pending mutation
observers until after we're finished with the "clone", because we can't
actually copy or move the intrusive list. As far as I am aware, this
should not affect behaviour because only one microtask can run at once.
It turns out it was a mistake to make this a virtual since
ServiceWorkerAgents are effectively the exact same as
DedicatedWorkerAgents and SharedWorkerAgents just with [[CanBlock]]
set to false.
To allow for adding the concept of a WorkerAgent to be reused
between shared and dedicated workers. An event loop is the
commonality between the different agent types, though, there
are some differences between those event loops which we customize
on the construction of the HTML::EventLoop.
