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

Source/modules/mediastream/MediaStreamTrackEvent.cpp

Index: Source/modules/mediastream/MediaStreamTrackEvent.cpp
diff --git a/Source/modules/mediastream/MediaStreamTrackEvent.cpp b/Source/modules/mediastream/MediaStreamTrackEvent.cpp
index 0a9b65f9e28443793d06d4c9ad4ddf694712c863..ac9078ad0869ea29757f1c509e27fc3f2c168301 100644
--- a/Source/modules/mediastream/MediaStreamTrackEvent.cpp
+++ b/Source/modules/mediastream/MediaStreamTrackEvent.cpp
@@ -67,5 +67,9 @@ const AtomicString& MediaStreamTrackEvent::interfaceName() const
     return EventNames::MediaStreamTrackEvent;
 }
 
-} // namespace WebCore
+void MediaStreamTrackEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
 
+} // namespace WebCore