Re: [webcomponents] <iframe> and Window's named properties (#145)

The basic rule should be:

- Do not leak any node, name or anything else about a shadow tree via any *existing* APIs. We do not want developers to access shadow trees unintentionally.
- It's okay to access a node in an open shadow tree via *new* API, e.g. `Element.shadowRoot`, `Element.getAssingedSlot`, `Event.deepPath()`, because there is an explicit intention.
- Never allow to access a closed shadow tree, even when an author uses a new API. e.g. `Element.shadowRoot` should return null for a closed shadow tree.

We might want to judge case by case carefully, however, we should honer this basic rule as possible as we can.


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