Oilpan: fix build after r187709.

Source/core/dom/ScopedWindowFocusAllowedIndicator.h

 // Copyright 2014 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 ScopedWindowFocusAllowedIndicator_h
 #define ScopedWindowFocusAllowedIndicator_h
 
 #include "core/dom/ContextLifecycleObserver.h"
 #include "core/dom/ExecutionContext.h"
 #include "wtf/Noncopyable.h"
@@ skipping 20 matching lines @@
         {
             if (executionContext)
                 executionContext->allowWindowFocus();
         }
 
         void dispose()
         {
             if (executionContext())
                 executionContext()->consumeWindowFocus();
         }
+
+        void trace(Visitor*) { }
     };
 
     // In Oilpan, destructors are not allowed to touch other on-heap objects.
     // The Observer indirection is needed to keep
     // ScopedWindowFocusAllowedIndicator off-heap and thus allows its destructor
     // to call executionContext()->consumeWindowFocus().
     OwnPtrWillBePersistent<Observer> m_observer;
 };
 
 } // namespace blink
 
 #endif // ScopedWindowFocusAllowedIndicator_h