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The Web Animations spec now uses the name AnimationPlaybackEvent instead of
AnimationPlayerEvent:
http://w3c.github.io/web-animations/#the-animationplaybackevent-interface

This patch also makes the timelineTime property of AnimationPlaybackEvent
nullable, as specced.

BUG=624639

Review-Url: https://codereview.chromium.org/2344473002
Cr-Commit-Position: refs/heads/master@{#419103}