fetch upstream change

Author: saschanaz

inputfiles/idl/Fullscreen.widl

@@ -25,6 +25,6 @@ partial interface Document {
   attribute EventHandler onfullscreenerror;
 };
 
-partial interface DocumentOrShadowRoot {
+partial interface mixin DocumentOrShadowRoot {
   [LenientSetter] readonly attribute Element? fullscreenElement;
 };