Fix flaky negative-playback-rate animation test.

third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html

Index: third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html
diff --git a/third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html b/third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html
index bc3422da131df048de4dc3fac5f14116638240e1..b854e163eb4d91a4f1f6bd98612730d28091a671 100644
--- a/third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html
+++ b/third_party/WebKit/LayoutTests/web-animations-api/animation-state-changes-negative-playback-rate.html
@@ -19,7 +19,7 @@ function idleAnimation() {
 function runningAnimation() {
   var animation = idleAnimation();
   animation.play();
-  animation.startTime = document.timeline.currentTime + duration;
+  animation.startTime = document.timeline.currentTime + duration / 2;
   return animation;
 }
 
@@ -60,7 +60,7 @@ test(function() {
 test(function() {
   var animation = runningAnimation();
   assert_equals(animation.startTime, document.timeline.currentTime - (animation.playbackRate * animation.currentTime));
-  assert_equals(animation.currentTime, duration);
+  assert_equals(animation.currentTime, duration / 2);
   assert_equals(animation.playState, 'running');
 }, "running");
 
@@ -204,7 +204,7 @@ test(function() {
   var animation = runningAnimation();
   animation.pause();
   assert_unresolved(animation.startTime);
-  assert_equals(animation.currentTime, duration);
+  assert_equals(animation.currentTime, duration / 2);
   assert_equals(animation.playState, 'pending');
 }, "running -> pause()");
 
@@ -228,7 +228,7 @@ test(function() {
   var animation = runningAnimation();
   animation.reverse();
   assert_unresolved(animation.startTime);
-  assert_equals(animation.currentTime, 0);
+  assert_equals(animation.currentTime, duration / 2);
   assert_equals(animation.playState, 'pending');
 }, "running -> reverse()");