[webcomponents] ownerDocument of a node in a shadow tree isn't well defined (#303)

`ownerDocument` of a node in a shadow tree is defined as follows:
> The ownerDocument property of a node in a shadow tree must refers to the document of the shadow host which hosts the shadow tree.

However, the definition of "the document of the shadow host" isn't clear.  I think we need to instead define  [node document](https://dom.spec.whatwg.org/#concept-node-document) for each node in a shadow tree.  That definition can be recursive and the [definition of `ownerDocument` property in DOM4](https://dom.spec.whatwg.org/#dom-node-ownerdocument) will take care of the rest.

But quite frankly, I'm not even sure we need to redefine what the node document of a node inside shadow tree will become because the node document is set as soon as a shadow tree is created, which is defined later in the spec.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/303