Web Animations: Add a runtime flag for playback control

This will guard the next shipping subset of the Web Animations API.

BUG=396369

Review URL: https://codereview.chromium.org/538503002

git-svn-id: svn://svn.chromium.org/blink/trunk@181696 bbb929c8-8fbe-4397-9dbb-9b2b20218538