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

Source/core/events/PageTransitionEvent.h

Index: Source/core/events/PageTransitionEvent.h
diff --git a/Source/core/events/PageTransitionEvent.h b/Source/core/events/PageTransitionEvent.h
index cc79580ba080e911998af3d4ad77c3208d723628..f249292a84b8d3a45aec6fc401eb52be2ae164f0 100644
--- a/Source/core/events/PageTransitionEvent.h
+++ b/Source/core/events/PageTransitionEvent.h
@@ -57,6 +57,8 @@ public:
 
     bool persisted() const { return m_persisted; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     PageTransitionEvent();
     PageTransitionEvent(const AtomicString& type, bool persisted);