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As a result of aligning the animation clock to raf, the animations in
play-state.html start a bit earlier. This resulted in the test flaking
regularly. By waiting for a render and relaxing the time allowed this
test no longer flakes. I've added another test to ensure that
animations which start with the initial document load are indeed
caught by an animationstart event.

Bug: 986019
Change-Id: Ib0dff60ddfef4f0395e422ac94bbdc78cc660017
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1727370Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684335}