layout: Stop exposing raw Element types to layout as well.

This should make layout memory-safe as is, though we will need to do
more stuff for incremental reflow and parallelism.

src/components/script/dom/document.rs

@@ -29,7 +29,6 @@ use std::cast;
 use std::hashmap::HashMap;
 use std::str::eq_slice;
 use std::unstable::raw::Box;
-use style::{TElement, TNode};
 
 #[deriving(Eq)]
 pub enum DocumentTypeId {