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

Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp

Index: Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp
diff --git a/Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp b/Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp
index 86b0e57bedcd1d1eab3de4dc10e59be27467051d..bb0ff3d860db76e54d3fb1a52759ca811e20f001 100644
--- a/Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp
+++ b/Source/modules/encryptedmedia/MediaKeyMessageEvent.cpp
@@ -57,4 +57,9 @@ const AtomicString& MediaKeyMessageEvent::interfaceName() const
     return EventNames::MediaKeyMessageEvent;
 }
 
+void MediaKeyMessageEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore