Re: [webcomponents] Disallow attachShadow(...) for some elements. (#110)

@treshugart that does not work, e.g., `<iframe>` was already mentioned as something we cannot allow this on.

@hayatoito why can we not include more elements we know are not problematic as I suggested in https://github.com/w3c/webcomponents/issues/110#issuecomment-148634611? I guess I don't feel particularly strongly though.

Also, we should use safelist/blocklist rather than whitelist/blacklist I think. See https://github.com/whatwg/html/issues/265 for rationale.

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