Made CSSPrimitiveValue::getString() only work for String types

Source/core/css/CSSPrimitiveValue.cpp

Index: Source/core/css/CSSPrimitiveValue.cpp
diff --git a/Source/core/css/CSSPrimitiveValue.cpp b/Source/core/css/CSSPrimitiveValue.cpp
index 95e9b3b0bbeebe21c38c7dcc41bac05f23bd0d1a..6bc0cf9f93d69d0921c516145bf44294f5c65376 100644
--- a/Source/core/css/CSSPrimitiveValue.cpp
+++ b/Source/core/css/CSSPrimitiveValue.cpp
@@ -748,14 +748,11 @@ String CSSPrimitiveValue::getStringValue() const
     case UnitType::Attribute:
     case UnitType::URI:
         return m_value.string;
-    case UnitType::ValueID:
-        return valueName(m_value.valueID);
-    case UnitType::PropertyID:
-        return propertyName(m_value.propertyID);
     default:
         break;
     }
 
+    ASSERT_NOT_REACHED();
     return String();
 }