Make anchor-pos scroll snapshot test pass for run-snapshot-post-layout

Let a style and layout update happen for a frame before expecting that
scroll snapshotting happened.

Preparing for proposed change:

whatwg/html#11613

Bug: 384523570
Change-Id: Ie0575420a686903efdae5e88c27e1c593518820c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6954082
Commit-Queue: Rune Lillesveen <[email protected]>
Reviewed-by: Morten Stenshorne <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1515992}

Author: lilles

css/css-anchor-position/anchor-scroll-js-expose.html

@@ -55,7 +55,11 @@
   scroller.scrollLeft = 1300;
 
   // Ensure up-to-date scroll snapshot.
-  await new Promise(resolve => requestAnimationFrame(resolve));
+  await new Promise(resolve => {
+    requestAnimationFrame(() => {
+      requestAnimationFrame(resolve);
+    });
+  });
 });
 
 promise_test(async () => {