We now evaluate the conditions of `@media` rules at the same point in
the HTML event loop as evaluation of `MediaQueryList`s. This is not
strictly to spec, but since the spec doesn't actually say when to do
this, it seems to make the most sense. In any case, it works! :^)
This patch makes both of these classes inherit from RefCounted and
Bindings::Wrappable, plus some minimal rejigging to allow us to keep
using them internally while also exposing them to web content.
