Web Animations: Add hasStarted to DocumentTimeline

This will be used in delaying animations from being dispatched to the
compositor until the corresponding timeline has started.

Review URL: https://codereview.chromium.org/71453006

git-svn-id: svn://svn.chromium.org/blink/trunk@161998 bbb929c8-8fbe-4397-9dbb-9b2b20218538