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/script_task.rs

@@ -52,7 +52,6 @@ use std::ptr;
 use std::str::eq_slice;
 use std::task::{spawn_sched, SingleThreaded};
 use std::util::replace;
-use style::{TElement, TNode};
 
 /// Messages used to control the script task.
 pub enum ScriptMsg {