Re: [webcomponents] Proposed change for Focus Navigation (#381)

@hayatoito PTAL.

I tried to get this upstreamable to HTML spec, [section 6.4](https://html.spec.whatwg.org/multipage/interaction.html#focus),
but it is hard to do in a single step, as the HTML spec describes the
navigation order as an algorithm to iteratively find the next focusable node,
while the Shadow DOM spec (before my modification) assumes there is a
(somewhat) pre-computed *focus navigation order* for each scope and
describes how they are merged together.

So this is my attempt to include the #375 as exactly as possible by
following the current way of describing focus navigation in Shadow DOM.

At this point it seems to me to patch the upstream spec requires lot more work.

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