Implement unset value handling

Source/core/css/CSSValuePool.h

Index: Source/core/css/CSSValuePool.h
diff --git a/Source/core/css/CSSValuePool.h b/Source/core/css/CSSValuePool.h
index 86068f5ef473782a6cd5270f8500f5bb8c09f7bd..97af05983b1c41e39e25a1d340a836020da37fda 100644
--- a/Source/core/css/CSSValuePool.h
+++ b/Source/core/css/CSSValuePool.h
@@ -31,6 +31,7 @@
 #include "core/css/CSSInheritedValue.h"
 #include "core/css/CSSInitialValue.h"
 #include "core/css/CSSPrimitiveValue.h"
+#include "core/css/CSSUnsetValue.h"
 #include "wtf/HashMap.h"
 #include "wtf/RefPtr.h"
 #include "wtf/text/AtomicStringHash.h"
@@ -47,6 +48,7 @@ public:
     PassRefPtrWillBeRawPtr<CSSInheritedValue> createInheritedValue() { return m_inheritedValue; }
     PassRefPtrWillBeRawPtr<CSSInitialValue> createImplicitInitialValue() { return m_implicitInitialValue; }
     PassRefPtrWillBeRawPtr<CSSInitialValue> createExplicitInitialValue() { return m_explicitInitialValue; }
+    PassRefPtrWillBeRawPtr<CSSUnsetValue> createUnsetValue() { return m_unsetValue; }
     PassRefPtrWillBeRawPtr<CSSPrimitiveValue> createIdentifierValue(CSSValueID identifier);
     PassRefPtrWillBeRawPtr<CSSPrimitiveValue> createIdentifierValue(CSSPropertyID identifier);
     PassRefPtrWillBeRawPtr<CSSPrimitiveValue> createColorValue(unsigned rgbValue);
@@ -64,6 +66,7 @@ private:
     RefPtrWillBeMember<CSSInheritedValue> m_inheritedValue;
     RefPtrWillBeMember<CSSInitialValue> m_implicitInitialValue;
     RefPtrWillBeMember<CSSInitialValue> m_explicitInitialValue;
+    RefPtrWillBeMember<CSSUnsetValue> m_unsetValue;
 
     WillBeHeapVector<RefPtrWillBeMember<CSSPrimitiveValue>, numCSSValueKeywords> m_identifierValueCache;