Explicitly mark first 2 args of addEventListener/removeEventListener as optional

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 7febedfcedc843658d302a0918b9c8853da896d5..26ceb1b3573d9b4a877efb3c43d6c352d636f3fc 100644
--- a/Source/core/events/EventTarget.idl
+++ b/Source/core/events/EventTarget.idl
@@ -21,11 +21,13 @@
 [
     Custom=ToV8,
 ] interface EventTarget {
-    void addEventListener([TreatNullAs=NullString] DOMString type,
-                          EventListener listener,
+    // FIXME: first 2 args should be required, but throwing TypeError breaks
+    // legacy content.  http://crbug.com/353484
+    void addEventListener([TreatNullAs=NullString] optional DOMString type,
+                          optional EventListener listener,
                           optional boolean useCapture);
-    void removeEventListener([TreatNullAs=NullString] DOMString type,
-                             EventListener listener,
+    void removeEventListener([TreatNullAs=NullString] optional DOMString type,
+                             optional EventListener listener,
                              optional boolean useCapture);
     [RaisesException] boolean dispatchEvent(Event event);
 };