Oilpan: fix build after r191643 (4e6a640c).

Source/modules/presentation/DefaultSessionStartEvent.h

Index: Source/modules/presentation/DefaultSessionStartEvent.h
diff --git a/Source/modules/presentation/DefaultSessionStartEvent.h b/Source/modules/presentation/DefaultSessionStartEvent.h
index 61f3d82a184ef0a530a6d1aaac41c9ed146d067e..fe779d75438bb4237db6564a09996fbb79de8125 100644
--- a/Source/modules/presentation/DefaultSessionStartEvent.h
+++ b/Source/modules/presentation/DefaultSessionStartEvent.h
@@ -38,12 +38,14 @@ public:
 
     virtual const AtomicString& interfaceName() const override;
 
+    DECLARE_VIRTUAL_TRACE();
+
 private:
     DefaultSessionStartEvent();
     DefaultSessionStartEvent(const AtomicString& eventType, PresentationSession*);
     DefaultSessionStartEvent(const AtomicString& eventType, const DefaultSessionStartEventInit& initializer);
 
-    Persistent<PresentationSession> m_session;
+    PersistentWillBeMember<PresentationSession> m_session;
 };
 
 DEFINE_TYPE_CASTS(DefaultSessionStartEvent, Event, event, event->interfaceName() == EventNames::DefaultSessionStartEvent, event.interfaceName() == EventNames::DefaultSessionStartEvent);