fixup! Use DOMRefCell<T> in element.rs.
Revert "fixup! Use DOMRefCell<T> in element.rs."

This reverts commit 145338fd94abc39ae6eb695dc5c5ef1712db516e.
Revert "Use DOMRefCell<T> in element.rs."

This reverts commit d1ae985322e6de6d0212012af142ad19eb6415da.
Revert "Add DOMRefCell<T> for safe borrowing in layout."

This reverts commit b095ab3fbc354846a1a7148890c4ab1933708017.
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.
DOMRefCell<T>.borrow_in_layout() returns &T instead of *const T.
Use mem::transmute() to cheat the lifetime restriction.