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

Source/core/events/ProgressEvent.cpp

Index: Source/core/events/ProgressEvent.cpp
diff --git a/Source/core/events/ProgressEvent.cpp b/Source/core/events/ProgressEvent.cpp
index d50bdad5edf8dc03b432c18269981707cec66889..0835160af95285647ba070bccfe4bfce38f7eead 100644
--- a/Source/core/events/ProgressEvent.cpp
+++ b/Source/core/events/ProgressEvent.cpp
@@ -68,4 +68,9 @@ const AtomicString& ProgressEvent::interfaceName() const
     return EventNames::ProgressEvent;
 }
 
+void ProgressEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 }