Fix animations not starting on detached elements

LayoutTests/virtual/threaded/animations/composited-animation-style-update.html

Index: LayoutTests/virtual/threaded/animations/composited-animation-style-update.html
diff --git a/LayoutTests/animations/composited-animation-style-update.html b/LayoutTests/virtual/threaded/animations/composited-animation-style-update.html
similarity index 55%
rename from LayoutTests/animations/composited-animation-style-update.html
rename to LayoutTests/virtual/threaded/animations/composited-animation-style-update.html
index 25f7a923af013aeecb4e8b7b9c1ee1015db12239..ebf1c51e89345b4717f26adf19cfcd0e3fc7bab1 100644
--- a/LayoutTests/animations/composited-animation-style-update.html
+++ b/LayoutTests/virtual/threaded/animations/composited-animation-style-update.html
@@ -17,15 +17,25 @@ div {
     background: green;
 }
 </style>
+<span id="text"></span>
 <div id="expectation"></div>
 <div id="target"></div>
 <script>
 target.style.fontSize = "50px";
-target.animate([
+var animation = target.animate([
   {transform: "translate(1em, 1em)"},
   {transform: "translate(1em, 1em)"},
 ], {
-  duration: 1000,
+  duration: 1e10,
   iterations: Infinity,
 });
+
+testRunner.waitUntilDone();
+requestAnimationFrame(function() {
+  requestAnimationFrame(function() {
+    var composited = internals.isCompositedAnimation(animation);
+    text.textContent = `Animation ${composited ? 'is' : 'is not'} running on the compositor.`;
+    testRunner.notifyDone();
+  });
+});
 </script>