Introduce toHTMLFormControlElement, and use it.

Source/core/editing/FrameSelection.cpp

Index: Source/core/editing/FrameSelection.cpp
diff --git a/Source/core/editing/FrameSelection.cpp b/Source/core/editing/FrameSelection.cpp
index f15e997a27ccfdb6fe8bd4087586884829171daf..6414c4db61ddcbe640572c0dd43b5aa0e52041fd 100644
--- a/Source/core/editing/FrameSelection.cpp
+++ b/Source/core/editing/FrameSelection.cpp
@@ -1963,7 +1963,7 @@ static HTMLFormElement* scanForForm(Node* start)
         if (element->hasTagName(formTag))
             return static_cast<HTMLFormElement*>(element);
         if (element->isHTMLElement() && toHTMLElement(element)->isFormControlElement())
-            return static_cast<HTMLFormControlElement*>(element)->form();
+            return toHTMLFormControlElement(element)->form();
         if (element->hasTagName(frameTag) || element->hasTagName(iframeTag)) {
             Node* childDocument = static_cast<HTMLFrameElementBase*>(element)->contentDocument();
             if (HTMLFormElement* frameResult = scanForForm(childDocument))
@@ -1987,7 +1987,7 @@ HTMLFormElement* FrameSelection::currentForm() const
         if (node->hasTagName(formTag))
             return static_cast<HTMLFormElement*>(node);
         if (node->isHTMLElement() && toHTMLElement(node)->isFormControlElement())
-            return static_cast<HTMLFormControlElement*>(node)->form();
+            return toHTMLFormControlElement(node)->form();
     }
 
     // Try walking forward in the node tree to find a form element.