Re: [webcomponents] Composition Algorithm shouldn't unwrap slots (#308)

I don't think we want to do this just for the styling use cases mentioned here.  I think we just need to set [`display: contents`](http://www.w3.org/TR/css-display-3/#box-generation) as the default sheet sheet on slot elements as follows and that should address both use cases mentioned here.

```css
slot { display: contents; }
```

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