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

Source/core/html/HTMLLegendElement.cpp

Index: Source/core/html/HTMLLegendElement.cpp
diff --git a/Source/core/html/HTMLLegendElement.cpp b/Source/core/html/HTMLLegendElement.cpp
index 0fb130d61405fed4135eabe8b68322a3064c8578..086f3381df45947cb40c2a5ed673539736311936 100644
--- a/Source/core/html/HTMLLegendElement.cpp
+++ b/Source/core/html/HTMLLegendElement.cpp
@@ -46,9 +46,7 @@ DEFINE_NODE_FACTORY(HTMLLegendElement)
 HTMLFormControlElement* HTMLLegendElement::associatedControl()
 {
     // Check if there's a fieldset belonging to this legend.
-    Element* fieldset = parentElement();
-    while (fieldset && !isHTMLFieldSetElement(*fieldset))
-        fieldset = fieldset->parentElement();
+    HTMLFieldSetElement* fieldset = Traversal<HTMLFieldSetElement>::firstAncestor(*this);
     if (!fieldset)
         return 0;