Explicitly mark first 2 args of addEventListener/removeEventListener as optional

Source/core/events/EventTarget.cpp

Index: Source/core/events/EventTarget.cpp
diff --git a/Source/core/events/EventTarget.cpp b/Source/core/events/EventTarget.cpp
index 99ef9227f2564d76dcd9030b3703718fef889d88..7243a6ec75f6ddac686223fc914fde927cfc83a2 100644
--- a/Source/core/events/EventTarget.cpp
+++ b/Source/core/events/EventTarget.cpp
@@ -82,6 +82,10 @@ inline DOMWindow* EventTarget::executingWindow()
 
 bool EventTarget::addEventListener(const AtomicString& eventType, PassRefPtr<EventListener> listener, bool useCapture)
 {
+    // FIXME: listener null check should throw TypeError (and be done in
+    // generated bindings), but breaks legacy content. http://crbug.com/249598
+    if (!listener)
+        return false;
     EventListener* eventListener = listener.get();
     if (ensureEventTargetData().eventListenerMap.add(eventType, listener, useCapture)) {
         InspectorInstrumentation::didAddEventListener(this, eventType, eventListener, useCapture);