Introduce toHTMLFormControlElement, and use it.

Source/core/html/HTMLFieldSetElement.cpp

Index: Source/core/html/HTMLFieldSetElement.cpp
diff --git a/Source/core/html/HTMLFieldSetElement.cpp b/Source/core/html/HTMLFieldSetElement.cpp
index d441041217c29d78515751eb4d0bf92de1af230c..0a77f073be2a5f626d230fe7a8a35a70f54ebff7 100644
--- a/Source/core/html/HTMLFieldSetElement.cpp
+++ b/Source/core/html/HTMLFieldSetElement.cpp
@@ -54,7 +54,7 @@ void HTMLFieldSetElement::invalidateDisabledStateUnder(Element* base)
 {
     for (Element* element = ElementTraversal::firstWithin(base); element; element = ElementTraversal::next(element, base)) {
         if (element->isFormControlElement())
-            static_cast<HTMLFormControlElement*>(element)->ancestorDisabledStateWasChanged();
+            toHTMLFormControlElement(element)->ancestorDisabledStateWasChanged();
     }
 }
 
@@ -123,7 +123,7 @@ void HTMLFieldSetElement::refreshElementsIfNeeded() const
         if (!element->isFormControlElement())
             continue;
 
-        m_associatedElements.append(static_cast<HTMLFormControlElement*>(element));
+        m_associatedElements.append(toHTMLFormControlElement(element));
     }
 }