update externs for 2017-era implementations

[samthor]

• moves *Timeline interfaces into the main externs
• makes Animation a constructor, not an interface (i.e., we didn't allow new Animation)
• adds id, including making AnimationEffectTimingProperties a @record so it can be extended by KeyframeAnimationOptions and used on element.animate (which returns an Animation with id).

I've also made it clear that the -next externs are just for the polyfill, and that they don't match the spec.

Plus some misc cleanups (e.g. remove opt_ from optional arguments—no longer required by Closure).

[shans]

@samthor is this still worth looking at?

[samthor]

If the Web Animations API hasn't changed, which I don't think it has, then yes. The current version doesn't express the whole API available in Chrome, and folks using Closure might struggle.