Re: [whatwg/fullscreen] Removing `:fullscreen-ancestor` pseudo-class causes webcompat issue (#103)

Whether the ancestor elements are visible doesn't matter, but whether they exist matters. The page has rules like
```css
:-moz-full-screen-ancestor>:not(:-moz-full-screen-ancestor):not(:-moz-full-screen) {
 display:none!important
}
/* as well as those with other vendor prefixes */
```
so when in fullscreen, if `:-moz-full-screen-ancestor` is valid, everything outside the fullscreen element would no longer generate any box, which means the page basically has no height, and thus `scrollY` becomes zero, which triggers some change inside the fullscreen element (hide some element there so that video is visible).

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