VT: Deflake nothing captured view transition test.

Instead of relying on a double rAF, use waitForCompositorReady in
an attempt to deflake the test.

Also update the takeScreenshot condition to await the ready
promise instead of a double raf.

R=kevers@chromium.org

Bug: 378536381, 378637954
Change-Id: Icc54a556d7b26d9d74bb9b7fd39a25aef6adccd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6041322
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386391}

Author: vmpstr

css/css-view-transitions/nothing-captured.html

@@ -5,6 +5,7 @@
 <link rel="author" href="mailto:vmpstr@chromium.org">
 <link rel="match" href="nothing-captured-ref.html">
 <script src="/common/reftest-wait.js"></script>
+<script src="/dom/events/scrolling/scroll_support.js"></script>
 <style>
 :root { view-transition-name: none; }
 .target {
@@ -39,9 +40,14 @@
 failIfNot(document.startViewTransition, "Missing document.startViewTransition");
 
 async function runTest() {
-  document.startViewTransition(() =>
-    requestAnimationFrame(() => requestAnimationFrame(takeScreenshot)));
+  await document.startViewTransition().ready;
+  requestAnimationFrame(takeScreenshot);
 }
-onload = () => requestAnimationFrame(() => requestAnimationFrame(runTest));
+
+onload = async () => {
+  await waitForCompositorReady();
+  runTest();
+}
+
 </script>