[w3c/webcomponents] importNode and cloneNode should upgrade cloned resolved custom elements (#584)

When a cloned custom element had already been well defined in the current global object, `importNode` and `cloneNode` should immediately upgrade the cloned elements.

Otherwise, we get into a bizarre situation like calling `cloneNode` on a synchronously constructed or upgraded custom element isn't another instance of the same custom element until it gets inserted into a document.  That is, `(new CustomElement).cloneNode() instanceof CustomElement` is `false`.  That's just silly.

I'm already [getting a bug report about this](https://bugs.webkit.org/show_bug.cgi?id=163147) in WebKit.

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