Introduce CSS_VALUE_TYPE_CASTS macro to cast CSSValue

Source/core/css/resolver/ElementStyleResources.cpp

Index: Source/core/css/resolver/ElementStyleResources.cpp
diff --git a/Source/core/css/resolver/ElementStyleResources.cpp b/Source/core/css/resolver/ElementStyleResources.cpp
index f99919aeb2790cf0727a8f2e2d8b40f0b0cb23b4..6534ea87d7b751a154fd3e28856da1b29b0f402b 100644
--- a/Source/core/css/resolver/ElementStyleResources.cpp
+++ b/Source/core/css/resolver/ElementStyleResources.cpp
@@ -50,7 +50,7 @@ PassRefPtr<StyleImage> ElementStyleResources::styleImage(const TextLinkColors& t
     }
 
     if (value->isImageSetValue())
-        return setOrPendingFromValue(property, static_cast<CSSImageSetValue*>(value));
+        return setOrPendingFromValue(property, toCSSImageSetValue(value));
 
     if (value->isCursorImageValue())
         return cursorOrPendingFromValue(property, static_cast<CSSCursorImageValue*>(value));