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

Source/core/accessibility/AXNodeObject.cpp

 /*
 * Copyright (C) 2012, Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 333 matching lines @@
 {
     if (!element->isHTMLElement() || !toHTMLElement(element)->isLabelable())
         return 0;
 
     const AtomicString& id = element->getIdAttribute();
     if (!id.isEmpty()) {
         if (HTMLLabelElement* label = element->treeScope().labelElementForId(id))
             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
 {
     Element* menuItem = menuItemElementForMenu();
 
     if (menuItem) {
         // ARIA just has generic menu items. AppKit needs to know if this is a top level items like MenuBarButton or MenuBarItem
         AXObject* menuItemAX = axObjectCache()->getOrCreate(menuItem);
         if (menuItemAX && menuItemAX->isMenuButton())
@@ skipping 1334 matching lines @@
     float range = maxValueForRange() - minValueForRange();
     float value = valueForRange();
 
     value += range * (percentChange / 100);
     setValue(String::number(value));
 
     axObjectCache()->postNotification(node(), AXObjectCache::AXValueChanged, true);
 }
 
 } // namespace blink