add 'slow' prefix to RenderObject's firstChild() / lastChild() methods

Source/core/rendering/TextAutosizer.h

Index: Source/core/rendering/TextAutosizer.h
diff --git a/Source/core/rendering/TextAutosizer.h b/Source/core/rendering/TextAutosizer.h
index 922dbc953f3bc5f055bc4da3c8304826dfe164d7..54504b4db0d683684bca7c71fb1e8d1d9540a9e3 100644
--- a/Source/core/rendering/TextAutosizer.h
+++ b/Source/core/rendering/TextAutosizer.h
@@ -118,7 +118,7 @@ private:
 
     // Depending on the traversal direction specified, finds the first or the last leaf text node child that doesn't
     // belong to any cluster.
-    static const RenderObject* findFirstTextLeafNotInCluster(const RenderObject*, size_t& depth, TraversalDirection);
+    static const RenderObject* findFirstTextLeafNotInCluster(const RenderBlock*, size_t& depth, TraversalDirection);
 
     // Returns groups of narrow descendants of a given autosizing cluster. The groups are combined
     // by the difference between the width of the descendant and the width of the parent cluster's