De-flake native-popup-with-datalist

I was able to reproduce the flake which was happening because sometimes
the button was still focused. I fixed it by blurring the focused element
and adding a rAF.

Fixed: 334122678
Change-Id: Iede088900b888ab32c3a9554d5c6fc83b980ccc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5530688
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300308}

html/semantics/forms/the-select-element/stylable-select/native-popup-with-datalist-ref.html

@@ -12,6 +12,8 @@
 <script>
 (async () => {
   await test_driver.click(document.querySelector('select'));
+  document.activeElement.blur();
+  await new Promise(requestAnimationFrame);
   document.documentElement.classList.remove('reftest-wait');
 })();
 </script>

html/semantics/forms/the-select-element/stylable-select/native-popup-with-datalist.tentative.html

@@ -20,6 +20,8 @@
 <script>
 (async () => {
   await test_driver.click(document.querySelector('select'));
+  document.activeElement.blur();
+  await new Promise(requestAnimationFrame);
   document.documentElement.classList.remove('reftest-wait');
 })();
 </script>