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.
Use DOMRefCell<T> in Attr.
Use DOMRefCell<T> in Element.
Use DOMRefCell<T> in CharacterData.
auto merge of #3574 : saneyuki/servo/cell, r=jdm

#3050