User agents must not support aria-owns across shadow root boundaries

[aleventhal]

Because aria-owns is notoriously difficult for user agents to implement in a stable and predictable manner, I suggest that aria-owns cannot cross shadow root boundaries.

Pointing into an ancestor shadow root is already something user agents would hopefully not allow, because it would create a cycle. I would also like to make it impossible to point down via ariaOwnsElements, referenceDelegate, or any other cross-shadow root ARIA technique.

This is my recommendation unless someone can come up with a legit use case.

@jcsteh @scottaohara @cookiecrook @alice