Introduce toHTMLFormControlElement, and use it.

Source/core/html/HTMLFormControlElement.h

Index: Source/core/html/HTMLFormControlElement.h
diff --git a/Source/core/html/HTMLFormControlElement.h b/Source/core/html/HTMLFormControlElement.h
index 27f8656b90e9bb5d445d37a02cb66344e402dba1..afdafdeb1f6d53062ab552dceb3f515488f203d0 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -171,6 +171,18 @@ private:
     bool m_hasAutofocused : 1;
 };
 
+inline HTMLFormControlElement* toHTMLFormControlElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || (node->isElementNode() && toElement(node)->isFormControlElement()));
+    return static_cast<HTMLFormControlElement*>(node);
+}
+
+inline HTMLFormControlElement* toHTMLFormControlElement(FormAssociatedElement* control)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!control || control->isFormControlElement());
+    return static_cast<HTMLFormControlElement*>(control);
+}
+
 } // namespace
 
 #endif