Web Animations CSS: Schedule a timer to wake DocumentTimeline if no short-term animation updates are required.

Currently, DocumentTimeline will repeatedly requestAnimationFrame() if future animations are pending, regardless of how long it is before those animations actually impact style.

This change ensures that if there are no immediate pending changes, a timer is set to wake DocumentTimeline before the appropriate frame instead.

BUG=258896

Review URL: https://codereview.chromium.org/34613007

git-svn-id: svn://svn.chromium.org/blink/trunk@160394 bbb929c8-8fbe-4397-9dbb-9b2b20218538