Group child initialization 

This should rectify this issue https://github.com/web-animations/web-animations-next/issues/260

@shans @dstockwell ptal
> Why did this move?
Hmm. This actually might be an issue. This is what I think is happening:

When you play a group, you document.timeline.play all of it's children. That makes a new maxifill player per child, and the maxifill player constructor (indirectly) calls _updateChildren with the child as the root. But then setting this and calling _updateChildren recursively resets everyone's player to be the top level player.

seems like a round about way to do it and I should probably fix it.