Use DEFINE_ELEMENT_TYPE_CASTS*() macros for Element subclasses

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 20a6e8f6b60e5fa27fbc9245a8bff1f0009b2313..889056f495f2b9130250e76b60caab5bf743a4b1 100644
--- a/Source/core/html/HTMLLegendElement.cpp
+++ b/Source/core/html/HTMLLegendElement.cpp
@@ -57,13 +57,7 @@ HTMLFormControlElement* HTMLLegendElement::associatedControl()
 
     // Find first form element inside the fieldset that is not a legend element.
     // FIXME: Should we consider tabindex?
-    Element* element = fieldset;
-    while ((element = ElementTraversal::next(*element, fieldset))) {
-        if (element->isFormControlElement())
-            return toHTMLFormControlElement(element);
-    }
-
-    return 0;
+    return Traversal<HTMLFormControlElement>::next(*fieldset, fieldset);
 }
 
 void HTMLLegendElement::focus(bool, FocusType type)