Introduce DEFINE_CONTENT_DATA_TYPE_CASTS, and use it

Source/core/css/resolver/StyleResourceLoader.cpp

Index: Source/core/css/resolver/StyleResourceLoader.cpp
diff --git a/Source/core/css/resolver/StyleResourceLoader.cpp b/Source/core/css/resolver/StyleResourceLoader.cpp
index c9652dfcec7818231554dbee2545f084aee9d9cf..16ee44d79b401ae09a9e5c0356d29444ff893a19 100644
--- a/Source/core/css/resolver/StyleResourceLoader.cpp
+++ b/Source/core/css/resolver/StyleResourceLoader.cpp
@@ -134,11 +134,11 @@ void StyleResourceLoader::loadPendingImages(RenderStyle* style, ElementStyleReso
         case CSSPropertyContent: {
             for (ContentData* contentData = const_cast<ContentData*>(style->contentData()); contentData; contentData = contentData->next()) {
                 if (contentData->isImage()) {
-                    StyleImage* image = static_cast<ImageContentData*>(contentData)->image();
+                    StyleImage* image = toImageContentData(contentData)->image();
                     if (image->isPendingImage()) {
                         RefPtr<StyleImage> loadedImage = loadPendingImage(toStylePendingImage(image), elementStyleResources.deviceScaleFactor());
                         if (loadedImage)
-                            static_cast<ImageContentData*>(contentData)->setImage(loadedImage.release());
+                            toImageContentData(contentData)->setImage(loadedImage.release());
                     }
                 }
             }