Bug 1264125 part 2 - Add ontransitioncancel EventHandler to WebIDL. r=baku

We moved 'transitioncancel' event to the CSS-Transition Level 1.
The definition of 'transitioncancel' is as follow:
https://drafts.csswg.org/css-transitions-1/#transitioncancle

MozReview-Commit-ID: 62s9R7MpXmk

--HG--
extra : rebase_source : 1774410824f32b1f822b83ccec3f132b6dad77de

dom/webidl/EventHandler.webidl

@@ -131,6 +131,7 @@ interface GlobalEventHandlers {
            attribute EventHandler onanimationend;
            attribute EventHandler onanimationiteration;
            attribute EventHandler onanimationstart;
+           attribute EventHandler ontransitioncancel;
            attribute EventHandler ontransitionend;
            attribute EventHandler ontransitionrun;
            attribute EventHandler ontransitionstart;