It holds a function pointer to the HeapSizeOf::heap_size_of_children()
implementation corresponding to that IDL interface.
This removes the need for a clumsly TypeId-based match expression in the
former heap_size_of_eventtarget() function.
Some interfaces like Node, CharacterData and HTMLTableCellElement are never
instantiated directly, only their descendant interfaces are. Those are marked
with [Abstract] to set their type_id to None instead of having dummy values
in the TypeId enums.
