Web Animations API: Ship Element.animate

As proposed on blink-dev[1]. Note this doesn't flip any virtual/stable tests
as a canary version of animate was enabled in r171017[2].

[1] https://groups.google.com/a/chromium.org/d/msg/blink-dev/SWxBI0uPvUE/iZCNwoN3nKEJ
[2] https://src.chromium.org/viewvc/blink?revision=171017&view=revision

BUG=360469

Review URL: https://codereview.chromium.org/248223003

git-svn-id: svn://svn.chromium.org/blink/trunk@172342 bbb929c8-8fbe-4397-9dbb-9b2b20218538