Implement unset value handling

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 24 matching lines @@
 CSSValuePool& cssValuePool()
 {
     DEFINE_STATIC_LOCAL(OwnPtrWillBePersistent<CSSValuePool>, pool, (adoptPtrWillBeNoop(new CSSValuePool())));
     return *pool;
 }
 
 CSSValuePool::CSSValuePool()
     : m_inheritedValue(CSSInheritedValue::create())
     , m_implicitInitialValue(CSSInitialValue::createImplicit())
     , m_explicitInitialValue(CSSInitialValue::createExplicit())
+    , m_unsetValue(CSSUnsetValue::create())
     , m_colorTransparent(CSSPrimitiveValue::createColor(Color::transparent))
     , m_colorWhite(CSSPrimitiveValue::createColor(Color::white))
     , m_colorBlack(CSSPrimitiveValue::createColor(Color::black))
 {
     m_identifierValueCache.resize(numCSSValueKeywords);
     m_pixelValueCache.resize(maximumCacheableIntegerValue + 1);
     m_percentValueCache.resize(maximumCacheableIntegerValue + 1);
     m_numberValueCache.resize(maximumCacheableIntegerValue + 1);
 }
 
@@ skipping 107 matching lines @@
     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
 }
 
 }