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

Source/modules/mediastream/MediaStreamEvent.cpp

Index: Source/modules/mediastream/MediaStreamEvent.cpp
diff --git a/Source/modules/mediastream/MediaStreamEvent.cpp b/Source/modules/mediastream/MediaStreamEvent.cpp
index bd401619c4782ee26d475e64ee451cf41302712c..874bcb80317be01ae79aebb8fff385a9f4d532b6 100644
--- a/Source/modules/mediastream/MediaStreamEvent.cpp
+++ b/Source/modules/mediastream/MediaStreamEvent.cpp
@@ -88,5 +88,10 @@ const AtomicString& MediaStreamEvent::interfaceName() const
     return EventNames::MediaStreamEvent;
 }
 
+void MediaStreamEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore