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.

components/layout/wrapper.rs

@@ -552,6 +552,13 @@ impl<'le> TElement<'le> for LayoutElement<'le> {
         }
     }
 
+    #[inline]
+    fn get_checked_state(self) -> bool {
+        unsafe {
+            self.element.get_checked_state_for_layout()
+        }
+    }
+
     #[inline]
     fn has_class(self, name: &Atom) -> bool {
         unsafe {