Make the setCustomValidity() error argument not nullable

Source/core/html/HTMLButtonElement.idl

Index: Source/core/html/HTMLButtonElement.idl
diff --git a/Source/core/html/HTMLButtonElement.idl b/Source/core/html/HTMLButtonElement.idl
index 3509851e990ed497da7b1d9802ef21ca2a1a4724..f3c5787d937ba6569c0cb8e2a3c8bda27a571f34 100644
--- a/Source/core/html/HTMLButtonElement.idl
+++ b/Source/core/html/HTMLButtonElement.idl
@@ -39,8 +39,7 @@ interface HTMLButtonElement : HTMLElement {
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
-    // FIXME: The error argument should not be nullable.
-    void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
+    void setCustomValidity(DOMString? error);

[philipj_slow, 2015/07/02 09:53:00]

You should also remove the "?", that's what makes the argument nullable. Gecko
has this in their IDL files:
void setCustomValidity(DOMString error);

The cases that need to be tested are then setCustomValidity(null) and
setCustomValidity(undefined), which should result in strings "null" and
"undefined" wherever it is that the error argument ends up.

 
     readonly attribute NodeList labels;
 };