selectors: Add parsing support for ::slotted().

Without turning it on yet, of course.

The reason why I didn't use the general PseudoElement mechanism is because this
pseudo is a bit of its own thing, and I found easier to make ::selectors know
about it (because you need to jump to the assigned slot) than the other way
around.

Also, we need to support ::slotted(..)::before and such, and supporting multiple
pseudo-elements like that breaks some other invariants around the SelectorMap,
and fixing those would require special-casing slotted a lot more in other parts
of the code.

Let me know if you think otherwise.

I also don't like much the boolean tuple return value, but I plan to do some
cleanup in the area in a bit, so it should go away soon, I'd hope.

components/selectors/builder.rs

@@ -264,12 +264,23 @@ fn complex_selector_specificity<Impl>(mut iter: slice::Iter<Component<Impl>>)
                                       -> Specificity
     where Impl: SelectorImpl
 {
-    fn simple_selector_specificity<Impl>(simple_selector: &Component<Impl>,
-                                         specificity: &mut Specificity)
-        where Impl: SelectorImpl
+    fn simple_selector_specificity<Impl>(
+        simple_selector: &Component<Impl>,
+        specificity: &mut Specificity,
+    )
+    where
+        Impl: SelectorImpl
     {
         match *simple_selector {
             Component::Combinator(..) => unreachable!(),
+            // FIXME(emilio): Spec doesn't define any particular specificity for
+            // ::slotted(), so apply the general rule for pseudos per:
+            //
+            // https://github.com/w3c/csswg-drafts/issues/1915
+            //
+            // Though other engines compute it dynamically, so maybe we should
+            // do that instead, eventually.
+            Component::Slotted(..) |
             Component::PseudoElement(..) |
             Component::LocalName(..) => {
                 specificity.element_selectors += 1