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

Source/core/html/MediaKeyEvent.h

Index: Source/core/html/MediaKeyEvent.h
diff --git a/Source/core/html/MediaKeyEvent.h b/Source/core/html/MediaKeyEvent.h
index f6726a25e816050033eed5d9944e8cfc6d82132a..fb7112743a813613525fb369eeda0a2f71855faa 100644
--- a/Source/core/html/MediaKeyEvent.h
+++ b/Source/core/html/MediaKeyEvent.h
@@ -68,6 +68,8 @@ public:
     MediaKeyError* errorCode(bool& isNull) const { isNull = !m_errorCode; return m_errorCode.get(); }
     unsigned short systemCode() const { return m_systemCode; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaKeyEvent();
     MediaKeyEvent(const AtomicString& type, const MediaKeyEventInit& initializer);