Bug 1109390 part 4 - Make DevTools animation actor wait for asynchronous pause; r=pbrosset

Although pause() is not yet asynchronous, any time we finish calling it the
ready promise should be resolved so we can safely wait on the ready promise
after calling pause already. This way, once pause() becomes async later in this
bug, code relying on this actor will continue to work.

toolkit/devtools/server/actors/animation.js

@@ -250,6 +250,7 @@ let AnimationPlayerActor = ActorClass({
    */
   pause: method(function() {
     this.player.pause();
+    return this.player.ready;
   }, {
     request: {},
     response: {}
@@ -513,10 +514,13 @@ let AnimationsActor = exports.AnimationsActor = ActorClass({
    * Pause all animations in the current tabActor's frames.
    */
   pauseAll: method(function() {
+    let readyPromises = [];
     for (let player of this.getAllAnimationPlayers()) {
       player.pause();
+      readyPromises.push(player.ready);
     }
     this.allAnimationsPaused = true;
+    return promise.all(readyPromises);
   }, {
     request: {},
     response: {}