Web Animations: Avoid flakiness in change-in-animation-frame test

We were relying on the test completing within 10ms.

BUG=587711

Review URL: https://codereview.chromium.org/1705103004

Cr-Commit-Position: refs/heads/master@{#376128}

third_party/WebKit/LayoutTests/web-animations-api/change-in-animation-frame.html

@@ -8,8 +8,8 @@ if (window.testRunner) {
 }
 
 requestAnimationFrame(function() {
-  document.getElementById('pass').animate([{visibility: 'visible'}, {visibility: 'visible'}], 10);
-  document.getElementById('fail').animate([{visibility: 'hidden'}, {visibility: 'hidden'}], 10);
+  document.getElementById('pass').animate([{visibility: 'visible'}, {visibility: 'visible'}], {fill: 'forwards'});
+  document.getElementById('fail').animate([{visibility: 'hidden'}, {visibility: 'hidden'}], {fill: 'forwards'});
   if (window.testRunner) {
     testRunner.notifyDone();
   }