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

Source/modules/mediastream/RTCDataChannelEvent.cpp

Index: Source/modules/mediastream/RTCDataChannelEvent.cpp
diff --git a/Source/modules/mediastream/RTCDataChannelEvent.cpp b/Source/modules/mediastream/RTCDataChannelEvent.cpp
index 42da9cf879623215e854b8c8a87c74fdbee4df70..d2c3fda83c6f744ad01ee0536c2de6faef9a42f0 100644
--- a/Source/modules/mediastream/RTCDataChannelEvent.cpp
+++ b/Source/modules/mediastream/RTCDataChannelEvent.cpp
@@ -66,5 +66,9 @@ const AtomicString& RTCDataChannelEvent::interfaceName() const
     return EventNames::RTCDataChannelEvent;
 }
 
-} // namespace WebCore
+void RTCDataChannelEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
 
+} // namespace WebCore