Implement unset value handling

Source/core/css/CSSUnsetValue.cpp

Index: Source/core/css/CSSUnsetValue.cpp
diff --git a/Source/core/css/resolver/StyleResolverParentScope.cpp b/Source/core/css/CSSUnsetValue.cpp
similarity index 62%
copy from Source/core/css/resolver/StyleResolverParentScope.cpp
copy to Source/core/css/CSSUnsetValue.cpp
index 76cae0e07a77e9aac748018e46172c4601020be3..e3e8903d4be940ecfa827358c319d40ba74dfb64 100644
--- a/Source/core/css/resolver/StyleResolverParentScope.cpp
+++ b/Source/core/css/CSSUnsetValue.cpp
@@ -3,11 +3,15 @@
 // found in the LICENSE file.
 
 #include "config.h"
+#include "core/css/CSSUnsetValue.h"
 
-#include "core/css/resolver/StyleResolverParentScope.h"
+#include "wtf/text/WTFString.h"
 
 namespace blink {
 
-StyleResolverParentScope* StyleResolverParentScope::s_currentScope = 0;
+String CSSUnsetValue::customCSSText() const
+{
+    return "unset";
+}
 
 } // namespace blink