Web Animations CSS: Only trigger CSS animation events on animation frame updates

No visible change in behavior. This will enable jitter in the timeline time
between a regular task and an animation-frame task if necessary. This also
enables events to work as expected once we allow access to AnimationPlayers 
generated for CSS Animations and Transitions to be exposed via the API.

Review URL: https://codereview.chromium.org/270063003

git-svn-id: svn://svn.chromium.org/blink/trunk@173453 bbb929c8-8fbe-4397-9dbb-9b2b20218538