Add const versions of some toHTMLFooElement().

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 d9a64f53c67d2c447f816266bf602e2dab8e4ed4..2189f95a3b09a0f20cc1b7e36d3bdd9a14729aaa 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -182,6 +182,12 @@ inline HTMLFormControlElement* toHTMLFormControlElement(FormAssociatedElement* c
     return static_cast<HTMLFormControlElement*>(control);
 }
 
+inline const HTMLFormControlElement* toHTMLFormControlElement(const FormAssociatedElement* control)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!control || control->isFormControlElement());
+    return static_cast<const HTMLFormControlElement*>(control);
+}
+
 } // namespace
 
 #endif