[whatwg/fullscreen] Re-introduce the hierarchy restrictions (#140)

The hierarchy restrictions were removed in 766dc872bcf26c012549f7bdfde21c46bebba40c and 9592913bbeca3a9d029274f310eb27ad8003622c because it seemed to be unnecessary after we moved to the top layer model.

However, it forms an unfortunate combination with the change to make `fullscreenchange` event dispatched to element, that if we have multiple leaf fullscreen element in the stack, some of the element wouldn't get the `fullscreenchange` event. I raised in [#89 (comment)](https://github.com/whatwg/fullscreen/issues/89#issuecomment-302381330), and @foolip said he's fine adding back these restrictions in [#89 (comment)](https://github.com/whatwg/fullscreen/issues/89#issuecomment-302389588) and opened #91 to do so.

Recently, #91 was abandoned probably because of lack of activity and people don't recall the reasoning to do so. So I'm opening this issue to track that.

Blink currently doesn't have the hierarchy restrictions, while Gecko still has. And if we decide to re-introduce it, Blink may want to change before it's too late.

cc @dtapuska 

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