Re: [webcomponents] [Shadow] activeElement behavior seems to break encapsulation (#358)

After thinking of https://github.com/w3c/webcomponents/issues/358#issuecomment-180629067 more, I come to think if we have `HTMLSlotElement.activeElement` then it might be okay for `ShadowRoot.activeElement` to a slot within its shadow DOM.

I was imagining the following case, and the code is listening `mosuedown` event on shadow root, then uses `ShadowRoot.activeElement` to get where the focus is.  If it returns null, the code has to check the exact focused element via `document.activeElement` and recursive `ShadowRoot.activeElement`, then check if the element is under its own slot.

```html
<my-menu>
  #shadow-root
    <my-item>...</my-item>
    <my-item>...</my-item>
    <!-- optional items go here -->
    <slot></slot>
  #shadow-root-end
  <my-optional-item>...</my-optional-item>
  <my-optional-item>...</my-optional-item>
  <my-optional-item>...</my-optional-item>
</my-menu>
```

The concern is that to implement `ShadowRoot.activeElement` that way (returns slot), the UA native code has to do the similar search which may be a bit costly.

What do you think?

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