[w3c/webcomponents] shadow tree -> iframe <- contentWindow.frameElement encapsulation issue (#507)

We provide a custom element that contains an iframe in a shadow tree.  Before the iframe content loads, we register custom elements and ensure our API is globally available to the child document.  For example's sake, it looks something like this:

```html
<x-foo src="bar.html">
    #shadow-root
        <iframe src="bar.html"></iframe>
</x-foo>
```

As a closed tree, the user can't pierce the shadow tree from outside `<x-foo>`, but from inside _bar.html_, the user can access `window.frameElement`.  I think this is a mistake, as it undermines the encapsulation that the closed tree provides.

As a solution, I'd like to propose that if the iframe element is inside a shadow tree, `window.frameElement` returns the host.

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