Implements the :checked pseudo-class for inputs

Relevant spec:
https://html.spec.whatwg.org/multipage/scripting.html#selector-checked

Also modifies HTMLInputElement::SetChecked to no longer modify its
checked content value, instead making use of its internal checkedness
state now that we can match `:checked` properly.
auto merge of #4190 : mttr/servo/checked_pseudo_class, r=Manishearth

Relevant spec:
https://html.spec.whatwg.org/multipage/scripting.html#selector-checked

Also modifies HTMLInputElement::SetChecked to no longer modify its
checked content value, instead making use of its internal checkedness
state now that we can match `:checked` properly.