Add support for transitionrun events

These events are triggered as soon as a transition is added to the list
of running transitions. This will allow better test coverage while
reworking the transitions and animations processing model.

components/script/dom/webidls/EventHandler.webidl

@@ -92,6 +92,7 @@ interface mixin GlobalEventHandlers {
 
 // https://drafts.csswg.org/css-transitions/#interface-globaleventhandlers-idl
 partial interface mixin GlobalEventHandlers {
+           attribute EventHandler ontransitionrun;
            attribute EventHandler ontransitionend;
            attribute EventHandler ontransitioncancel;
 };