[webcomponents] It is unclear how tree of tress whose root tree is not a document tree should behave. (#356)

After I replaced "node tree" with "component tree" in https://github.com/w3c/webcomponents/commit/588ecdf984deba5a42b9d2b22e750fe96583b42d, I realized that it would affect the behavior of "tree of trees", in a sense before the commit, whose root tree is not a document tree.

The current spec does not say anything how such a *disconnected* (from a document) tree of trees should behave.

e.g.
```html
const host = document.createElement('div');
const child = document.createElement('div');
const shadowRoot = host.attachShadow({mode: 'open'});
shadowRoot.appendChild(document.createElement('slot'));
// Note that host is not inserted into a document.
console.log(child.assignedSlot);  // What should be?
```

FYI. in current Blink's implementation, the distribution is correctly calculated even for such a disconnected  *tree of trees*.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/356