Wait a full frame for the scroll timeline to become inactive

Modified scroll-animation-inactive-timeline.html test to allow for
snapshotting happening after first style/layout update.

See proposal in: whatwg/html#11613

Bug: 384523570
Change-Id: I44320e55325206e308d86b2cfd60668202cfe4a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7141918
Reviewed-by: Kevin Ellis <[email protected]>
Commit-Queue: Rune Lillesveen <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1543754}

Author: lilles

scroll-animations/scroll-timelines/scroll-animation-inactive-timeline.html

@@ -36,6 +36,7 @@
     // Wait for new animation frame which allows the timeline to compute new
     // current time.
     await waitForNextFrame();
+    await waitForNextFrame();
     assert_equals(animation.timeline.currentTime, null,
                   'Sanity check the timeline is inactive.');
     // Play the animation when the timeline is inactive.
@@ -62,6 +63,7 @@
     // Wait for new animation frame which allows the timeline to compute new
     // current time.
     await waitForNextFrame();
+    await waitForNextFrame();
     assert_equals(animation.timeline.currentTime, null,
                   'Sanity check the timeline is inactive.');
     // Play the animation when the timeline is inactive.
@@ -91,6 +93,7 @@
     // Wait for new animation frame which allows the timeline to compute new
     // current time.
     await waitForNextFrame();
+    await waitForNextFrame();
     assert_equals(animation.timeline.currentTime, null,
                   'Sanity check the timeline is inactive.');
     // Set start time when the timeline is inactive.
@@ -138,6 +141,7 @@
     scroller.style.overflow = 'visible';
     scroller.scrollTop;
     await waitForNextFrame();
+    await waitForNextFrame();
     assert_equals(animation.timeline.currentTime, null,
                   'Sanity check the timeline is inactive.');
     assert_equals(animation.playState, 'running',