Introduce CSS_VALUE_TYPE_CASTS macro to cast CSSValue

Source/core/rendering/style/StylePendingImage.h

Index: Source/core/rendering/style/StylePendingImage.h
diff --git a/Source/core/rendering/style/StylePendingImage.h b/Source/core/rendering/style/StylePendingImage.h
index 3633d154d1e5a57d30a05c0b8f7824cee311f32b..dd5acc70739c6b588147979f73228722c5716cac 100644
--- a/Source/core/rendering/style/StylePendingImage.h
+++ b/Source/core/rendering/style/StylePendingImage.h
@@ -49,7 +49,7 @@ public:
     CSSImageValue* cssImageValue() const { return m_value->isImageValue() ? toCSSImageValue(m_value) : 0; }
     CSSImageGeneratorValue* cssImageGeneratorValue() const { return m_value->isImageGeneratorValue() ? static_cast<CSSImageGeneratorValue*>(m_value) : 0; }
     CSSCursorImageValue* cssCursorImageValue() const { return m_value->isCursorImageValue() ? static_cast<CSSCursorImageValue*>(m_value) : 0; }
-    CSSImageSetValue* cssImageSetValue() const { return m_value->isImageSetValue() ? static_cast<CSSImageSetValue*>(m_value) : 0; }
+    CSSImageSetValue* cssImageSetValue() const { return m_value->isImageSetValue() ? toCSSImageSetValue(m_value) : 0; }
 
     virtual LayoutSize imageSize(const RenderObject*, float /*multiplier*/) const OVERRIDE { return LayoutSize(); }
     virtual bool imageHasRelativeWidth() const { return false; }