[webcomponents] Consider making ShadowRoot directly inherit from Node (#325)

It seems strange that `ShadowRoot` inherits from `DocumentFragment` given that `ShadowRoot` could be in the document whereas `DocumentFragment` can't traditionally be in the document ever.

Given our preferred way of adding methods to multiple interfaces is defining interfaces like `NonElementParentNode` and `ParentNode` and make other interfaces implement them, it's totally fine for `ShadowRoot` to inherit directly from `Node` and then make it implement everything `DocumentFragment` implements.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/325