Fix snav-focusless-fullscreen-video.html on mac

This is a speculative fix.

Bug: 979565
Change-Id: I140d9a3fa2334f7137e1491c40c61d96bcded8b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729922
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684427}

third_party/blink/web_tests/virtual/focusless-spat-nav/fast/spatial-navigation/focusless/snav-focusless-fullscreen-video.html

@@ -12,6 +12,9 @@
 <script>
   let video = document.getElementById("video");
   let button = document.getElementById("button");
+  let fullscreened = new Promise((resolve) => {
+    document.addEventListener('fullscreenchange', resolve);
+  });
 
   button.addEventListener('click', function() {
     video.requestFullscreen();
@@ -23,6 +26,13 @@
 
     await snavCallback();
 
+    await fullscreened;
+
+    // snav state racily may have changed once or twice when fullscreen state
+    // changes. Waiting until a rAF after fullscreen changes should give snav
+    // state time to update properly.
+    await snav.rAF();
+
     assert_equals(window.internals.interestedElement,
                   video,
                   "Video element should have interest.");