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

Source/modules/webmidi/MIDIMessageEvent.h

Index: Source/modules/webmidi/MIDIMessageEvent.h
diff --git a/Source/modules/webmidi/MIDIMessageEvent.h b/Source/modules/webmidi/MIDIMessageEvent.h
index b80744584c5caf8a557fd1b41ffdcdeb2a006d95..cfb29ea205ce8eef94c2ad6e605e57d7414259ed 100644
--- a/Source/modules/webmidi/MIDIMessageEvent.h
+++ b/Source/modules/webmidi/MIDIMessageEvent.h
@@ -68,6 +68,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::MIDIMessageEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { Event::trace(visitor); }
+
 private:
     MIDIMessageEvent()
         : m_receivedTime(0)