Address timeouts in composited CSS animations event dispatch test

There are no restrictions on how quickly the client agent needs to
respond to a pending play-state change. For non-composited animations
the ready promise is resolved on commit of the animation frame;
however, for a composited animation, the process is delayed until
receiving a start-time notification from the compositor. The event
dispatch test previously waited up to 2 animations frames, which is
adequate for non-composited tests since the events trigger on the next
frame. For composited tests, we need to first wait for the animation
to be ready if in a pending state. Once the ready promise is resolved,
a two frame time limit is sufficient for the remainder of the steps.

Tested with 300 repetitions of the flaking tests in the virtual/threaded
environment.

Bug: 1064065
Change-Id: Ic4975d5885d3513c5c1b53d755757c0adcc027c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341803Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795918}

third_party/blink/web_tests/external/wpt/css/css-animations/event-dispatch.tentative.html

@@ -18,12 +18,14 @@
 
 const setupAnimation = (t, animationStyle) => {
   const div = addDiv(t, { style: 'animation: ' + animationStyle });
+  const animation = div.getAnimations()[0];
+  const timeoutPromise = armTimeoutWhenReady(animation, fastEventsTimeout);
+
   const watcher = new EventWatcher(t, div, [ 'animationstart',
                                              'animationiteration',
                                              'animationend',
                                              'animationcancel' ],
-                                   fastEventsTimeout);
-  const animation = div.getAnimations()[0];
+                                   timeoutPromise);
 
   return { animation, watcher, div };
 };

third_party/blink/web_tests/external/wpt/css/css-animations/support/testcommon.js

@@ -201,6 +201,21 @@ function fastEventsTimeout() {
   return waitForAnimationFrames(2);
 };
 
+/**
+ * Timeout function used for tests with EventWatchers. The client agent has no
+ * strict requirement for how long it takes to resolve the ready promise. Once
+ * the promise is resolved a secondary timeout promise is armed that may have
+ * a tight deadline measured in animation frames.
+ */
+function armTimeoutWhenReady(animation, timeoutPromise) {
+  return () => {
+    if (animation.pending)
+      return animation.ready.then(() => { return timeoutPromise(); });
+    else
+      return timeoutPromise();
+  };
+}
+
 /**
  * Wrapper that takes a sequence of N animations and returns:
  *