Group child initialization 

This should rectify this issue https://github.com/web-animations/web-animations-next/issues/260

@shans @dstockwell ptal
> What's the order of operations for something like this?
> 
> var player = timeline.play(group);
> player.pause();

I'm not sure what you mean. In this case you will play the group, and all of it's child players will be created (via _updateChildren). Then the whole lot get's paused.

This case was the purpose of this change.