Web Animations: Compositor timeOffset should be sensitive to playback rate

ManualTests/animation/compositor-reverse.html

+<style>
+div {
+  height: 100px;
+  width: 100px;
+  background: blue;
+}
+</style>
+<p>
+The two squares should <strong>smoothly</strong> rock backwards and forwards.
+<div id="target1"></div>
+<div id="target2"></div>
+<script>
+var player1 = target1.animate([
+  {transform: 'none'},
+  {transform: 'rotate(90deg)'}
+], 1000);
+
+var player2 = target2.animate([
+  {transform: 'none', background: 'blue'},
+  {transform: 'rotate(90deg)', background: 'blue'}
+], 1000);
+
+setInterval(function() {
+  player1.reverse();
+  player2.reverse();
+}, 100);
+</script>