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 414056f14f4ea532b5623f39e230b67021bdbc22..bcf12717c54e0bf41b70c9834aa301f6232a2009 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();
 }