[fullscreen] Exit fullscreen on document with multiple subdocument in fullscreen (#30)

Assuming we have tree like:
```
body (document)
|
+- iframe #fa
|  |
|  +- div #a
|
+- iframe #fb
   |
   +- div #b
```

If we do:

1. `a.requestFullscreen()`
2. `b.requestFullscreen()`
3. `document.exitFullscreen()`

According to the current spec, it seems the result is:

* the content documents of both iframes exit fullscreen
* the current fullscreen element of the document becomes iframe #fa 

This doesn't seem to be correct.

I think only the content document of #fb should exit fullscreen.

I suggest moving the unfullscreen descendant document steps to **unfullscreen an element**.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/30