Web Animations API: Update element.animate and Player to reflect latest spec

Rename Player to AnimationPlayer
(http://lists.w3.org/Archives/Public/public-fx/2014JanMar/0083.html).
Convert element.animate to return an AnimationPlayer rather than an Animation
(http://lists.w3.org/Archives/Public/public-fx/2014JanMar/0099.html).

BUG=334926,327564

Review URL: https://codereview.chromium.org/196483004

git-svn-id: svn://svn.chromium.org/blink/trunk@169326 bbb929c8-8fbe-4397-9dbb-9b2b20218538