Sync web-platform-tests PR 28342 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/28342
Details from upstream follow.

b'Xianzhu Wang <wangxianzhu@chromium.org>' wrote:

Try to deflake css/css-animations/nested-scale-animations.html

• Reduce the size and the scale to avoid maximum scale clamping (which
  is the mainn reason of the test failing under virtual/threaded on
  Chrome).
• Use 'forwards' instead of 'infinite' for the animation, so that in
  case of capture happening after the end of the animation, the test
  will still pass.
• Wait for start of the animation, to avoid the test ending before the
  animation starts.
• Increase the delay of capturing to be near the end of the animation,
  to avoid the capture happening during animation from scale 1 to 10.

Bug: 1169154
Change-Id: I28e4dd6300377cf5524681f85ac099611ea617e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2798031
Commit-Queue: Xianzhu Wang \<wangxianzhu@chromium.org>
Reviewed-by: Stephen Chenney \<schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868601}

CI Results

Ran 15 Firefox configurations based on mozilla-central, and Firefox, Chrome, and Safari on GitHub CI

Total 1 tests

Status Summary

Firefox

PASS: 1

Chrome

PASS: 1

Safari

FAIL: 1

Links

Gecko CI (Treeherder)
GitHub PR Head
GitHub PR Base

https://hg.mozilla.org/mozilla-central/rev/168f6422e962