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

Source/modules/webmidi/MIDIConnectionEvent.h

Index: Source/modules/webmidi/MIDIConnectionEvent.h
diff --git a/Source/modules/webmidi/MIDIConnectionEvent.h b/Source/modules/webmidi/MIDIConnectionEvent.h
index a8804863d05a241b5890909b13da0afa3813e254..de2c53beb95a36fbac3f4acbdd2088e80783f0df 100644
--- a/Source/modules/webmidi/MIDIConnectionEvent.h
+++ b/Source/modules/webmidi/MIDIConnectionEvent.h
@@ -66,6 +66,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::MIDIConnectionEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { Event::trace(visitor); }
+
 private:
     MIDIConnectionEvent()
     {