This was last changed in 0257af7 and seems to have originated in e6fa2d6.

After 0b7c994 and 766dc87 there are no other hierarchy restrictions, and there's probably no reason to keep the iframe ancestor check. By itself, it doesn't seem to provide any interesting guarantees.

Motivation: Removing this may make it easier to propagate the result of the fullscreen element ready check to other processes, currently being discussed in https://bugs.chromium.org/p/chromium/issues/detail?id=402376#c13

@upsuper