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`.
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.
