Give snap events a frame to be dispatched

Attempt at fixing flakiness in scroll-state-snapped-snap-changing.html

Bug: 418263609
Change-Id: I0408d8cd8c5fc728eef34deac447d5acc245b6aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6582801
Reviewed-by: Daniil Sakhapov 
Commit-Queue: Rune Lillesveen 
Cr-Commit-Position: refs/heads/main@{#1464761}

Author: lilles

css/css-conditional/container-queries/scroll-state/scroll-state-snapped-snap-changing.html

@@ -57,6 +57,7 @@
         .pointerMove(0, 0, {origin: "viewport"})
         .pause(50)
         .send();
+    await waitForAnimationFrames(2);
     assert_equals(changeTarget, snapped1, "snapchange has not been called for #snapped2");
     assert_equals(changingTarget, snapped2, "snapchanging has been called for #snapped2");
     assert_equals(getComputedStyle(target1).getPropertyValue("--snapped"), "",