[whatwg/fullscreen] Reinstate the hierarchy restrictions (#91)

This reinstates parts of the fullscreen element ready check removed in
commit 766dc872bcf26c012549f7bdfde21c46bebba40c and
commit 9592913bbeca3a9d029274f310eb27ad8003622c, with some
simplification of the iframe ancestor check.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fullscreen/pull/91

-- Commit Summary --

  * Reinstate the hierarchy restrictions

-- File Changes --

    M fullscreen.bs (6)

-- Patch Links --

https://github.com/whatwg/fullscreen/pull/91.patch
https://github.com/whatwg/fullscreen/pull/91.diff

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