Re: [webcomponents] [Shadow] Need equivalent of ownerDocument for Elements in a shadow tree to get to their ShadowRoot (#315)

@domenic : In WebKit and Blink, it would require tree walking only in the case of a detached subtree that doesn't have a ShadowRoot.  Nodes that are in a document and/or in a shadow tree can return its root node in O(1).  I'd imagine some engines could always compute it in O(1).

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https://github.com/w3c/webcomponents/issues/315#issuecomment-137605654