Re: [webcomponents] Figure out terminology for Shadow DOM that everyone agrees on (#382)

I was trying to figure out how as e.g., an `<iframe>` inside a shadow tree, you know your host element got inserted into a document. That led me to conclude that https://github.com/whatwg/dom/issues/34 needs to be updated not only to notify descendants, but also nodes inside hosted shadow trees.

I'm still trying to figure out what the best primitive is to have there (if we need a distinct notification next to parent change or if ancestor change is sufficient for all nodes, basically, I guess it doesn't matter much).

If we add shadow trees to the mix though that ancestor becomes a shadow-host-including inclusive ancestor (or a inclusive composed ancestor per current Shadow DOM terminology). Is that still okay?

If the host element was inserted into a closed shadow tree (whose shadow-host-including inclusive ancestor was a document), what kind of events would be fired on the custom elements inside host element's shadow tree? We can't really tell them the ancestor that got inserted since that would leak the closed shadow tree. @domenic have you looked at these events yet for custom elements?

(Maybe this should be a distinct issue. On the other hand, I'm just trying to figure out what all the various primitives are since they're not really documented at the moment. And hopefully once we have the primitives we can figure out suitable terminology for them.)

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