Oilpan: fix build after r186604.

Source/core/css/CSSValuePool.cpp

 /*
  * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 138 matching lines @@
     }
     return value;
 }
 
 void CSSValuePool::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_inheritedValue);
     visitor->trace(m_implicitInitialValue);
     visitor->trace(m_explicitInitialValue);
+    visitor->trace(m_unsetValue);
     visitor->trace(m_identifierValueCache);
     visitor->trace(m_colorValueCache);
     visitor->trace(m_colorTransparent);
     visitor->trace(m_colorWhite);
     visitor->trace(m_colorBlack);
     visitor->trace(m_pixelValueCache);
     visitor->trace(m_percentValueCache);
     visitor->trace(m_numberValueCache);
     visitor->trace(m_fontFaceValueCache);
     visitor->trace(m_fontFamilyValueCache);
 #endif
 }
 
 }