Defer starting of animations until after compositing update

LayoutTests/transitions/webkit-clip-path-equality.html

Index: LayoutTests/transitions/webkit-clip-path-equality.html
diff --git a/LayoutTests/transitions/webkit-clip-path-equality.html b/LayoutTests/transitions/webkit-clip-path-equality.html
index a0ea565b52b95b31a7292dd29393fef559f82578..20a5be6cb33317a7bdad4e2ea3db5bb4b9b30362 100644
--- a/LayoutTests/transitions/webkit-clip-path-equality.html
+++ b/LayoutTests/transitions/webkit-clip-path-equality.html
@@ -13,17 +13,22 @@ if (window.testRunner) {
   testRunner.waitUntilDone();
 }
 var target = document.getElementById('target');
+var result = '';
 onload = function() {
   target.style.webkitClipPath = getComputedStyle(target).webkitClipPath;
+  requestAnimationFrame(step1);
+};
+function step1() {
   target.offsetTop;
-  var result = '';
   if (window.internals) {
     result = (internals.numberOfActiveAnimations() == 0) ?
         'PASS - No transition started.' :
         'FAIL - Unexpected transition started.';
   }
   target.style.webkitClipPath = 'circle(0px, 0px, 200px)';
-  target.offsetTop;
+  requestAnimationFrame(step2);
+}
+function step2() {
   if (window.internals) {
     result += (internals.numberOfActiveAnimations() == 1) ?
         '\nPASS - Transition started.' :