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

Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp

Index: Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp
diff --git a/Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp b/Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp
index 57e55a6dfc5b24b5eb19717233c92218f39301e0..a1cb24e9c3dc9868cc84d87f8ad0178ba923922a 100644
--- a/Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp
+++ b/Source/modules/mediastream/RTCDTMFToneChangeEvent.cpp
@@ -79,5 +79,10 @@ const AtomicString& RTCDTMFToneChangeEvent::interfaceName() const
     return EventNames::RTCDTMFToneChangeEvent;
 }
 
+void RTCDTMFToneChangeEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore