Bug 1506080 [wpt PR 13997] - Update interfaces/css-transitions.idl, a=testonly

Automatic update from web-platform-testsUpdate interfaces/css-transitions.idl (#13997)

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

wpt-commits: d7a9bb5578c95d8e92a7c67c7c91d603cabbb64e
wpt-pr: 13997

testing/web-platform/tests/interfaces/css-transitions.idl

@@ -17,7 +17,7 @@ dictionary TransitionEventInit : EventInit {
   CSSOMString pseudoElement = "";
 };
 
-partial interface GlobalEventHandlers {
+partial interface mixin GlobalEventHandlers {
   attribute EventHandler ontransitionrun;
   attribute EventHandler ontransitionstart;
   attribute EventHandler ontransitionend;