Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp

Index: Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp
diff --git a/Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp b/Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp
index 14ce95300fbae38f0a1e14100639caf12f4a317f..c8ebb7955008b957c2cc63ff033dd61981a108b9 100644
--- a/Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp
+++ b/Source/modules/encryptedmedia/MediaKeyNeededEvent.cpp
@@ -57,4 +57,9 @@ const AtomicString& MediaKeyNeededEvent::interfaceName() const
     return EventNames::MediaKeyNeededEvent;
 }
 
+void MediaKeyNeededEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore