[webcomponents] Disallow attachShadow on replaced elements (#312)

It seems that the behavior of attaching shadow roots on [replaced elements](https://html.spec.whatwg.org/#replaced-elements) such as img, embed, object, etc... are not well defined.  We should probably disallow it in v1.


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/312