Use Traversal<>::firstAncestor() API more in the code base

Source/core/accessibility/AXNodeObject.cpp

Index: Source/core/accessibility/AXNodeObject.cpp
diff --git a/Source/core/accessibility/AXNodeObject.cpp b/Source/core/accessibility/AXNodeObject.cpp
index 932711060a11db06ce160fa875015e169d91e8f5..ffb7bdea232c666899cc822006b56aed7ce28680 100644
--- a/Source/core/accessibility/AXNodeObject.cpp
+++ b/Source/core/accessibility/AXNodeObject.cpp
@@ -351,12 +351,7 @@ HTMLLabelElement* AXNodeObject::labelForElement(Element* element) const
             return label;
     }
 
-    for (Element* parent = element->parentElement(); parent; parent = parent->parentElement()) {
-        if (isHTMLLabelElement(*parent))
-            return toHTMLLabelElement(parent);
-    }
-
-    return 0;
+    return Traversal<HTMLLabelElement>::firstAncestor(*element);
 }
 
 AXObject* AXNodeObject::menuButtonForMenu() const