Defer starting of animations until after compositing update

LayoutTests/animations/compositor-start-event-timing.html

Index: LayoutTests/animations/compositor-start-event-timing.html
diff --git a/LayoutTests/animations/compositor-start-event-timing.html b/LayoutTests/animations/compositor-start-event-timing.html
index 4c26a2861ab72e7de67560eb32076b21ce3646d8..fad8325eb2fe4d2493cc81505f3caced882a4224 100644
--- a/LayoutTests/animations/compositor-start-event-timing.html
+++ b/LayoutTests/animations/compositor-start-event-timing.html
@@ -23,11 +23,13 @@ if (window.testRunner) {
   });
 }
 document.body.textContent = 'FAIL (expected if outside the virtual/threaded suite)';
-document.body.classList.add('start');
-document.body.offsetTop;
-setTimeout(function() {
-  document.body.addEventListener('webkitAnimationStart', function() {
-    document.body.textContent = 'PASS';
-  });
-}, 0);
+requestAnimationFrame(function() {
+  document.body.classList.add('start');
+  document.body.offsetTop;
+  setTimeout(function() {
+    document.body.addEventListener('webkitAnimationStart', function() {
+      document.body.textContent = 'PASS';
+    });
+  }, 0);
+});
 </script>