Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/events/EventTarget.idl

Index: Source/core/events/EventTarget.idl
diff --git a/Source/core/events/EventTarget.idl b/Source/core/events/EventTarget.idl
index 69a4bc18d7b6e8986ae9c3dbe76371cd5ae3ba73..c83100d24701c4bd417f8856c59ece3d6cc9fdad 100644
--- a/Source/core/events/EventTarget.idl
+++ b/Source/core/events/EventTarget.idl
@@ -24,10 +24,10 @@
 ] interface EventTarget {
     // FIXME: first 2 args should be required, but throwing TypeError breaks
     // legacy content.  http://crbug.com/353484
-    void addEventListener([TreatNullAs=NullString] optional DOMString type,
+    void addEventListener(optional DOMString? type,
                           optional EventListener listener,
                           optional boolean useCapture);
-    void removeEventListener([TreatNullAs=NullString] optional DOMString type,
+    void removeEventListener(optional DOMString? type,
                              optional EventListener listener,
                              optional boolean useCapture);
     [RaisesException] boolean dispatchEvent(Event event);