Fix Animation::playState

https://github.com/w3c/csswg-drafts/issues/5400

There is the issue that led to the spec change for play state:

https://github.com/w3c/csswg-drafts/issues/2066

The issue addresses undesirable toggling of the play state due to
inactive timelines, but does not seem to adequately address having an
unresolved (null) timeline.


Bug: 1113382
Change-Id: I33032ee1e9de10d3dce4958fce599bd98391328f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339676Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796403}

third_party/blink/renderer/core/animation/animation.cc

@@ -942,7 +942,9 @@ Animation::AnimationPlayState Animation::CalculateAnimationPlayState() const {
   //    * animation does not have either a pending play task or a pending pause
   //      task,
   //    then idle.
-  if (!CurrentTimeInternal() && !start_time_ && !PendingInternal())
+  // https://github.com/w3c/csswg-drafts/issues/5400
+  if (!CurrentTimeInternal() && (!start_time_ || !timeline_) &&
+      !PendingInternal())
     return kIdle;
 
   // 2. Either of the following conditions are true:

third_party/blink/web_tests/external/wpt/web-animations/timing-model/animations/setting-the-timeline-of-an-animation.html

@@ -60,7 +60,7 @@ test(t => {
     new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
                   null);
   animation.startTime = document.timeline.currentTime;
-  assert_equals(animation.playState, 'running');
+  assert_equals(animation.playState, 'idle');
 
   animation.timeline = document.timeline;
 
@@ -73,7 +73,7 @@ test(t => {
     new Animation(new KeyframeEffect(createDiv(t), null, 100 * MS_PER_SEC),
                   null);
   animation.startTime = document.timeline.currentTime - 200 * MS_PER_SEC;
-  assert_equals(animation.playState, 'running');
+  assert_equals(animation.playState, 'idle');
 
   animation.timeline = document.timeline;