Bug 1612639 - Fix flaky upstreamed Web Animations WPT; r=hiro

These tests assume that the startTime of the animation is calculated based on
the document current time at the moment the ready promise callback is run but
nothing in the spec requires that (only that it is calculated based on the
_ready time_).

Differential Revision: https://phabricator.services.mozilla.com/D61949

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/web-animations/timing-model/animations/playing-an-animation.html

@@ -144,33 +144,33 @@
   assert_equals(animation.startTime, null,
                 'Start time should be unresolved');
 
+  const playTime = animation.timeline.currentTime;
   animation.play();
   assert_true(animation.pending, 'Animation should be play-pending');
 
   await animation.ready;
 
   assert_false(animation.pending, 'animation should no longer be pending');
-  assert_times_equal(animation.startTime, animation.timeline.currentTime,
-                     'The start time of the playing animation should be set');
+  assert_greater_than(animation.startTime, playTime,
+                      'The start time of the playing animation should be set');
 }, 'Playing a canceled animation sets the start time');
 
 promise_test(async t => {
   const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
   animation.playbackRate = -1;
-  // animation.currentTime = 100 * MS_PER_SEC;
   animation.cancel();
   assert_equals(animation.startTime, null,
                 'Start time should be unresolved');
 
+  const playTime = animation.timeline.currentTime;
   animation.play();
   assert_true(animation.pending, 'Animation should be play-pending');
 
   await animation.ready;
 
   assert_false(animation.pending, 'Animation should no longer be pending');
-  assert_times_equal(animation.startTime,
-                     animation.timeline.currentTime + 100 * MS_PER_SEC,
-                     'The start time of a playing animation should be set');
+  assert_greater_than(animation.startTime, playTime + 100 * MS_PER_SEC,
+                      'The start time of the playing animation should be set');
 }, 'Playing a canceled animation backwards sets the start time');
 
 </script>