similar-origin window agents have the [[CanBlock]] flag set to false.
Achieve this by hooking up JS's concept with an agent to HTML::Agent.
For now, this is only hooked up to the similar-origin window agent
case but should be extended to the other agent types in the future.
This makes it more convenient to use the 'relvant agent' concept,
instead of the awkward dynamic casts we needed to do for every call
site.
mutation_observers is also changed to hold a GC::Root instead of raw
GC::Ptr. Somehow this was not causing problems before, but trips up CI
after these changes.
