Introduce toHTMLInputElement(Node*), and use it.

Source/core/html/HTMLInputElement.h

Index: Source/core/html/HTMLInputElement.h
diff --git a/Source/core/html/HTMLInputElement.h b/Source/core/html/HTMLInputElement.h
index 70187e4b32fa9bb971e15bebea12c82389a4c071..e7f6a19c0115c8dcf54f22e85df781a0cb8946a6 100644
--- a/Source/core/html/HTMLInputElement.h
+++ b/Source/core/html/HTMLInputElement.h
@@ -427,5 +427,21 @@ private:
     OwnPtr<ListAttributeTargetObserver> m_listAttributeTargetObserver;
 };
 
+inline HTMLInputElement* toHTMLInputElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::inputTag));
+    return static_cast<HTMLInputElement*>(node);
+}
+
+inline const HTMLInputElement* toHTMLInputElement(const Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::inputTag));
+    return static_cast<const HTMLInputElement*>(node);
+}
+
+// This will catch anyone doing an unnecessary cast.
+void toHTMLElement(const HTMLElement*);
+
+
 } //namespace
 #endif