Introduce toHTMLInputElement(Node*), and use it.

Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp

Index: Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp
diff --git a/Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp b/Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp
index f51bb674605c03eb76b529b2681e1cc4ce1f2fbe..dfb4dd5f01f54b3ccc1671a919c7f93def7d11df 100644
--- a/Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp
+++ b/Source/WebKit/chromium/src/DOMUtilitiesPrivate.cpp
@@ -43,7 +43,7 @@ namespace WebKit {
 bool elementHasLegalLinkAttribute(const Element* element, const QualifiedName& attrName)
 {
     if (attrName == srcAttr)
-        return element->hasTagName(imgTag) || element->hasTagName(scriptTag) || element->hasTagName(iframeTag) || element->hasTagName(frameTag) || (element->hasTagName(inputTag) && static_cast<const HTMLInputElement*>(element)->isImageButton());
+        return element->hasTagName(imgTag) || element->hasTagName(scriptTag) || element->hasTagName(iframeTag) || element->hasTagName(frameTag) || (element->hasTagName(inputTag) && toHTMLInputElement(element)->isImageButton());
     if (attrName == hrefAttr)
         return element->hasTagName(linkTag) || element->hasTagName(aTag) || element->hasTagName(areaTag);
     if (attrName == actionAttr)