Update interfaces/WebIDL.idl (#25049)

Source: https://github.com/tidoust/reffy-reports/blob/3b17e3d/ed/idl/WebIDL.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/718725371

Author: autofoolip

interfaces/WebIDL.idl

@@ -45,4 +45,4 @@ interface DOMException { // but see below note about ECMAScript binding
 
 typedef unsigned long long DOMTimeStamp;
 callback Function = any (any... arguments);
-callback VoidFunction = void ();
+callback VoidFunction = undefined ();