Introduce [Abstract] to mark non-leaf interfaces

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.

components/script/dom/webidls/CharacterData.webidl

@@ -10,6 +10,7 @@
  * liability, trademark and document use rules apply.
  */
 
+[Abstract]
 interface CharacterData : Node {
   [TreatNullAs=EmptyString] attribute DOMString data;
   readonly attribute unsigned long length;