[webcomponents] Update Section 6.2 Focus Navigation to reflect TPAC discussion (#375)

We should update the [section 6.2 Focus Navigation](http://w3c.github.io/webcomponents/spec/shadow/#focus-navigation) to reflect the discussion we had during TPAC last year.

I think what we want is akin to putting nodes inside a shadow DOM and a slot into its own [nested browsing context](https://html.spec.whatwg.org/#nested-browsing-context).  We should probably edit the HTML spec to create some sort of scoping mechanism, and make shadow DOM / slot element / browsing context use that concept.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/375