[webcomponents] attachShadow shouldn't have a fixed list of elements to throw (#309)

I don't think it's good to have a fixed list of element names for which we throw.  I'd prefer stating that we must throw whenever "binding" property is a user-agent-defined value, and then provide an informal note with an explicit list.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/309