Oilpan: fix build after r191643 (4e6a640c).

Source/modules/presentation/DefaultSessionStartEvent.cpp

 // 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.
 
 #include "config.h"
 #include "modules/presentation/DefaultSessionStartEvent.h"
 
 #include "modules/presentation/DefaultSessionStartEventInit.h"
 
 namespace blink {
@@ skipping 16 matching lines @@
     : Event(eventType, initializer)
     , m_session(initializer.session())
 {
 }
 
 const AtomicString& DefaultSessionStartEvent::interfaceName() const
 {
     return EventNames::DefaultSessionStartEvent;
 }
 
+DEFINE_TRACE(DefaultSessionStartEvent)
+{
+    visitor->trace(m_session);
+    Event::trace(visitor);
+}
+
 } // namespace blink