Re: [webcomponents] Introduce "delegatesFocus" flag to shadow root (#126)

Re `:focus` matching, it should be recursive.  e.g. in the following example,
`div#host1:focus` should match when either `input#outer` or `input#inner` is focused.
Of course `div#host2:focus` only matches when `input#inner` is focused.

```html
<div#host1>
  <:shadow>
     <div#host2>
        <:shadow>
           <input#inner>
        </:shadow>
    <input#outer>
  </:shadow>
</div>
```

Because a slot is not focusable, I don't think identical mechanism is necessary, but regarding to `:focus` matching I think it depends on the result of how we treat focus in slot for `.activeElement` (#358).

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