Revert "Clean up AttributeHandlers::get_attribute() for JS<Element>."

 - 216ffc165232930413bb0c8743de402d40e51093.
 - 658c80f3a3e006ab4e9a22483bbbf29986d619c8.
 - 7ed66c6cee87d9f9432589778a021a11c1e88dd3.