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

Even if a slot is a focusable element, I prefer `x-foo.shadowRoot.activeElement -> null` to `x-foo.shadowRoot.activeElement -> slot`.

As @kochi said in https://github.com/w3c/webcomponents/issues/358#issuecomment-179143107, in terms of a document tree, that would cause *two* active elements in a document tree, `focusable2` and `x-foo`. That will violate a lot of assumptions, I think.

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