Update interfaces/selection-api.idl

Source: https://github.com/tidoust/reffy-reports/blob/83bbd56/whatwg/idl/selection-api.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/452744285

interfaces/selection-api.idl

@@ -37,7 +37,7 @@ partial interface Window {
   Selection? getSelection();
 };
 
-partial interface GlobalEventHandlers {
+partial interface mixin GlobalEventHandlers {
   attribute EventHandler onselectstart;
   attribute EventHandler onselectionchange;
 };