Bug 1422995 - Use waitForNextFrame() instead of waitForFrame() to make sure the next requestAnimationFrame callback happen. r=birtles

With the conformant Promise handling (bug 1193394) and performing micro task
checkpoint in Animation tick (bug 1416966), if we call waitForFrame() inside
the callback for Animation.ready.then it will still be done in the same refresh
driver's tick.

MozReview-Commit-ID: GQJiDHHUlyD

--HG--
extra : rebase_source : 55813e6c1fc24193e0b4b1c87934debe80d357b5

dom/animation/test/css-animations/file_animations-dynamic-changes.html

@@ -26,7 +26,7 @@ promise_test(function(t) {
 
     // Wait a moment so we can confirm the startTime doesn't change (and
     // doesn't simply reflect the current time).
-    return waitForFrame();
+    return waitForNextFrame();
   }).then(function() {
     div.style.animationDuration = '200s';
     var animation = div.getAnimations()[0];