Have a concrete SelectorImpl type everywhere in the style crate.

It is conditionally compiled to one implementation or the other
(Gecko or Servo) with `#[cfg(…)]`.

components/style/dom.rs

@@ -144,18 +144,15 @@ pub trait TNode : Sized + Copy + Clone {
 
     /// Borrows the PrivateStyleData without checks.
     #[inline(always)]
-    unsafe fn borrow_data_unchecked(&self)
-        -> Option<*const PrivateStyleData<<Self::ConcreteElement as Element>::Impl>>;
+    unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData>;
 
     /// Borrows the PrivateStyleData immutably. Fails on a conflicting borrow.
     #[inline(always)]
-    fn borrow_data(&self)
-        -> Option<Ref<PrivateStyleData<<Self::ConcreteElement as Element>::Impl>>>;
+    fn borrow_data(&self) -> Option<Ref<PrivateStyleData>>;
 
     /// Borrows the PrivateStyleData mutably. Fails on a conflicting borrow.
     #[inline(always)]
-    fn mutate_data(&self)
-        -> Option<RefMut<PrivateStyleData<<Self::ConcreteElement as Element>::Impl>>>;
+    fn mutate_data(&self) -> Option<RefMut<PrivateStyleData>>;
 
     /// Get the description of how to account for recent style changes.
     fn restyle_damage(self) -> Self::ConcreteRestyleDamage;
@@ -176,9 +173,7 @@ pub trait TNode : Sized + Copy + Clone {
 
     /// Returns the style results for the given node. If CSS selector matching
     /// has not yet been performed, fails.
-    fn style(&self,
-             _context: &SharedStyleContext<<Self::ConcreteElement as Element>::Impl>)
-        -> Ref<Arc<ComputedValues>>
+    fn style(&self, _context: &SharedStyleContext) -> Ref<Arc<ComputedValues>>
         where <Self::ConcreteElement as Element>::Impl: SelectorImplExt {
         Ref::map(self.borrow_data().unwrap(), |data| data.style.as_ref().unwrap())
     }