Add DOMRefCell<T> for safe borrowing in layout.

This type simply wraps `RefCell<T>` to add the special method,
and introduce the method to return the pointer of the value contained in
itself, for used in layout task.

components/script/lib.rs

@@ -56,6 +56,7 @@ pub mod dom {
 
     /// The code to expose the DOM to JavaScript through IDL bindings.
     pub mod bindings {
+        pub mod cell;
         pub mod global;
         pub mod js;
         pub mod utils;