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

Source/core/events/ProgressEvent.h

Index: Source/core/events/ProgressEvent.h
diff --git a/Source/core/events/ProgressEvent.h b/Source/core/events/ProgressEvent.h
index 76bc750a844eec39f231959d746ccb7eccdc6497..337a9a89c4d37386ea3ddb2b899abff2b7462f53 100644
--- a/Source/core/events/ProgressEvent.h
+++ b/Source/core/events/ProgressEvent.h
@@ -59,6 +59,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 protected:
     ProgressEvent();
     ProgressEvent(const AtomicString& type, bool lengthComputable, unsigned long long loaded, unsigned long long total);