[web-animations-1] Add timeline to animate() options (#5013)

[majido]

Fixes #5013

The options dictionary in Element.animate now has an optional nullable timeline
member. This allows timeline to be set via that method in addition to Animation
constructor.

Here are some examples of how this may be used:

// Create animation with a null timeline.
div.animate({color: 'red' }, { duration: 1000, timeline: null});

// Create a scroll-linked animation.
div.animate({color: 'red' }, { duration: 1000, timeline: new ScrollTimeline()});

[birtles]

Looks good. Thanks!