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

Here are my points on #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.

`summary` element also seems to create a user-agent shadow root in [Blink](https://chromium.googlesource.com/chromium/blink/+/master/Source/core/html/HTMLSummaryElement.cpp) and [WebKit] (http://trac.webkit.org/browser/trunk/Source/WebCore/html/HTMLSummaryElement.cpp#L58) as well.

In addition, we may want to consider disallowing them on SVG tref and use elements.  In fact, we may want to consider allowing `attachShadow` only on `HTMLElement` and its subclasses.

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