Split out String, URI and CustomIdent from CSSPrimitiveValue

Source/core/css/CSSValue.h

Index: Source/core/css/CSSValue.h
diff --git a/Source/core/css/CSSValue.h b/Source/core/css/CSSValue.h
index f2e3db49dad6f4c1682a57e16f507841aea494af..9ecf8414a99e93cb99e159d81d4542d2e70cf791 100644
--- a/Source/core/css/CSSValue.h
+++ b/Source/core/css/CSSValue.h
@@ -87,6 +87,7 @@ public:
     bool isRadialGradientValue() const { return m_classType == RadialGradientClass; }
     bool isReflectValue() const { return m_classType == ReflectClass; }
     bool isShadowValue() const { return m_classType == ShadowClass; }
+    bool isStringValue() const { return m_classType == StringClass; }
     bool isCubicBezierTimingFunctionValue() const { return m_classType == CubicBezierTimingFunctionClass; }
     bool isStepsTimingFunctionValue() const { return m_classType == StepsTimingFunctionClass; }
     bool isLineBoxContainValue() const { return m_classType == LineBoxContainClass; }
@@ -117,6 +118,7 @@ protected:
         PrimitiveClass,
         CounterClass,
         QuadClass,
+        StringClass,
         ValuePairClass,
 
         // Image classes.