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

Source/core/html/track/TrackEvent.h

Index: Source/core/html/track/TrackEvent.h
diff --git a/Source/core/html/track/TrackEvent.h b/Source/core/html/track/TrackEvent.h
index 17fbf09b6c4ac1f5a6ad56cd4f9f31191555f619..15dc425464462c28cf2fe5fe2a8422864acd6ebf 100644
--- a/Source/core/html/track/TrackEvent.h
+++ b/Source/core/html/track/TrackEvent.h
@@ -55,6 +55,8 @@ public:
 
     TrackBase* track() const { return m_track.get(); }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     TrackEvent();
     TrackEvent(const AtomicString& type, const TrackEventInit& initializer);