Oilpan: fix build after r191643 (4e6a640c).

Source/modules/presentation/DefaultSessionStartEvent.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DefaultSessionStartEvent_h
 #define DefaultSessionStartEvent_h
 
 #include "modules/EventModules.h"
 #include "modules/presentation/PresentationSession.h"
 #include "platform/heap/Handle.h"
@@ skipping 20 matching lines @@
     }
     static PassRefPtrWillBeRawPtr<DefaultSessionStartEvent> create(const AtomicString& eventType, const DefaultSessionStartEventInit& initializer)
     {
         return adoptRefWillBeNoop(new DefaultSessionStartEvent(eventType, initializer));
     }
 
     PresentationSession* session() { return m_session.get(); }
 
     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);
 
 } // namespace blink
 
 #endif // DefaultSessionStartEvent_h