Fix test flakiness in reverse-running-animation

The animation was often finishing before the screenshot resulting in a
failure. Extended the duration of the animation and starting the
animation from the midpoint should address the flakiness without
needlessly extending the run time of the test.

Bug: 1029543
Change-Id: I32e7794ff77a3b26e7d7f260f80d4b00bbc9d6f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986212Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728253}

third_party/blink/web_tests/external/wpt/web-animations/timing-model/animations/reverse-running-animation.html

@@ -5,6 +5,7 @@
 <title>reverse running animation</title>
 <link rel="match" href="reverse-running-animation-ref.html">
 <script src="/common/reftest-wait.js"></script>
+<script src="../../testcommon.js"></script>
 <style>
   #box, #overlay {
     /* Add a border to ensure that anti-aliasing does not leak blue pixels
@@ -44,24 +45,25 @@
   </p>
 </body>
 <script>
-  onload = function() {
+  onload = async function() {
     // Double rAF to ensure that we are not bogged down during initialization
     // and the compositor is ready.
-    requestAnimationFrame(() => {
-      requestAnimationFrame(() => {
+    waitForAnimationFrames(2).then(() => {
       const elem = document.getElementById('box');
       const anim = elem.animate([
           { transform: 'translateX(100px)' },
-            { transform: 'translateX(100px)' },
-            { transform: 'translateX(200px)' },
+          { transform: 'translateX(100px)', offset: 0.49 },
+          { transform: 'translateX(200px)', offset: 0.51 },
           { transform: 'translateX(200px)' }
         ], {
-            duration: 1000
+          duration: 10000,
         });
-        setTimeout(() => {
+      anim.ready.then(() => {
+        anim.currentTime = 5000;
+        requestAnimationFrame(() => {
           anim.reverse();
-        }, 500);
-        takeScreenshotDelayed(900);
+          takeScreenshotDelayed(1000);
+        });
       });
     });
   };