Fixed web-animations-visualization.js's use of document.animationTimeline and use of play() in demo_example.html

I fixed the mistake in `web-animations-visualization.js` where it uses an outdated method of playing animations.

I also fixed fixed the outdated use of `play()` in `demo_example.html`.

This is meant to fix [#208](https://github.com/web-animations/web-animations-js/issues/208)