[w3c/webcomponents] [Template Instantiation] Review the cloning step (#740)

One of the steps in the [Template Instantiation](https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Template-Instantiation.md) is to clone **HTMLTemplateElement#content** using **Node#cloneNode**. But in this way, the result won't be upgraded until it's attached to the document. 

On the other hand, if cloning is done using **Document#ImportNode**, the result is cloned and upgraded appropriately.

This can become undesirable, for example, by using custom elements. With the current method, some lifecycles and initialization methods may be deferred.

To be taken as an example,
https://jsbin.com/pobavohufu/edit?js,console

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https://github.com/w3c/webcomponents/issues/740