Introduce DEFINE_CONTENT_DATA_TYPE_CASTS, and use it

Source/core/rendering/RenderBlock.cpp

Index: Source/core/rendering/RenderBlock.cpp
diff --git a/Source/core/rendering/RenderBlock.cpp b/Source/core/rendering/RenderBlock.cpp
index 9c91ba9dbd7ac85bec2bf97c2704ce2edb638ffc..7a4595189220fd889b048f09a08b345a08f23f86 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -204,10 +204,8 @@ static void appendImagesFromStyle(Vector<ImageResource*>& images, RenderStyle& b
     appendLayers(images, blockStyle.maskLayers());
 
     const ContentData* contentData = blockStyle.contentData();
-    if (contentData && contentData->isImage()) {
-        const ImageContentData* imageContentData = static_cast<const ImageContentData*>(contentData);
-        appendImageIfNotNull(images, imageContentData->image());
-    }
+    if (contentData && contentData->isImage())
+        appendImageIfNotNull(images, toImageContentData(contentData)->image());
     if (blockStyle.boxReflect())
         appendImageIfNotNull(images, blockStyle.boxReflect()->mask().image());
     appendImageIfNotNull(images, blockStyle.listStyleImage());