Re: [whatwg/fullscreen] Allow fullscreen on already fullscreen elements without user activation (#193)

Yes, good catch that we need to avoid seamless fullscreen transition between two different origins.

Perhaps we can say: `elem.requestFullscreen()` will require either:

- transient user activation (as we currently have in [Step 5 here](https://fullscreen.spec.whatwg.org/#dom-element-requestfullscreen)), or
- `Document.fullscreenElement.ownerDocument === elem.ownerDocument`.

@foolip What do you think?

Any chance an easier answer lies in the "relevant global object" in Step 5 above?  The call `elem.requestFullscreen()` cannot be made from a cross-origin `Window` because that `Window` can't access `elem`---at least in Chrome.  I don't know if this site isolation behavior is fully spec-ed.  @domenic?

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