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

Source/core/xml/XMLHttpRequestProgressEvent.h

Index: Source/core/xml/XMLHttpRequestProgressEvent.h
diff --git a/Source/core/xml/XMLHttpRequestProgressEvent.h b/Source/core/xml/XMLHttpRequestProgressEvent.h
index f6365158c1bd13bb93b9ff817972bc9d6af8c2cd..5e7aa4608ca6df92ce794c732bf0d2468d7eb171 100644
--- a/Source/core/xml/XMLHttpRequestProgressEvent.h
+++ b/Source/core/xml/XMLHttpRequestProgressEvent.h
@@ -49,6 +49,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::XMLHttpRequestProgressEvent; }
 
+    virtual void trace(Visitor* visitor) OVERRIDE { ProgressEvent::trace(visitor); }
+
 private:
     XMLHttpRequestProgressEvent()
     {