Verifies that element.requestFullscreen() call from a cross-origin subframe without user
activation works if and only if the top frame has user activation and it delegates the capability
to the subframe.
https://wicg.github.io/capability-delegation/spec.html
See wpt/html/user-activation/propagation*.html for child->parent user activation visibility tests.
TODO: Check same-origin iframes, sibling frames, and popup<->opener delegation.