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

Source/core/events/ResourceProgressEvent.h

Index: Source/core/events/ResourceProgressEvent.h
diff --git a/Source/core/events/ResourceProgressEvent.h b/Source/core/events/ResourceProgressEvent.h
index bb8ae58a4e643956e5c03e2ca2ee5e32eced0876..596baf13b05c58d5b8facc99a094ad9c210d0b5f 100644
--- a/Source/core/events/ResourceProgressEvent.h
+++ b/Source/core/events/ResourceProgressEvent.h
@@ -57,6 +57,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 protected:
     ResourceProgressEvent();
     ResourceProgressEvent(const AtomicString& type, bool lengthComputable, unsigned long long loaded, unsigned long long total, const String& url);