Web Animations: Ship playback control

As discussed on blink-dev[1].

[1] https://groups.google.com/a/chromium.org/d/topic/blink-dev/gfifgxHdPXU/discussion

BUG=396369

Review URL: https://codereview.chromium.org/568253002

git-svn-id: svn://svn.chromium.org/blink/trunk@181950 bbb929c8-8fbe-4397-9dbb-9b2b20218538