[whatwg/dom] ShadowRoot's delegatesFocus (#931)

@sideshowbarker discovered that Chrome exposes `delegatesFocus` on `ShadowRoot`. This hasn't really appeared in any specification as far as I can tell (e.g., dglazkov-era's https://www.w3.org/TR/2015/WD-shadow-dom-20151215/#the-shadowroot-interface doesn't have it), but given that we expose other state that can be set when creating a shadow root, such as `mode`, it might well be reasonable to add.

@rniwa @smaug---- @rakina thoughts?

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