Fix Flaky Composite Order test

This patch fixes the flakiness in getting running-animation order by
making sure the animation is running before we make a measurement.

This test passed all 500 repeated run.

Bug: 1060216
Change-Id: I8b271e1e0d31dac0203aa3a7bb3359f7f1d1eec6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099474Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Hao Sheng <haozhes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749528}

third_party/blink/web_tests/external/wpt/css/css-animations/CSSAnimation-compositeOrder.tentative.html

@@ -42,9 +42,9 @@ promise_test(async t => {
   const animA = div.animate({margin: ["100px","100px"]}, 100000);
   assert_equals(getComputedStyle(div).marginLeft, '100px');
   div.style.animation = 'margin50 100s';
-  assert_equals(getComputedStyle(div).marginLeft, '50px');
   // Wait for animation starts
-  await waitForAnimationFrames(2);
+  await animA.ready;
+  await waitForAnimationFrames(1);
   assert_equals(getComputedStyle(div).marginLeft, '100px',
                 "A higher-priority animation is not overriden by a more recent"
                 + "one.");