Figure out relationship with activation

[Manishearth]

See also: https://github.com/immersive-web/navigation/issues/5

Filing this early, but basically we may need to figure out the relationship of this spec with activation.

As detailed in https://github.com/immersive-web/webxr/issues/779#issuecomment-536116004, one direction we may end up going with is giving carte blanche activation to immersive sessions, or certainly having some relaxed notion of activation for immersive sessions.

That's less compatible with navigation: We shouldn't be carrying over activation to other sites one navigates to, since it may have been a misclick that you wish to undo. It's hard granting activation in immersive sessions since any action you may do could be a part of the interaction.

This ties in to trusted navigation UI (https://github.com/immersive-web/navigation/issues/5): do we want lightweight consent dialogs for navigation (e.g. blur the newly created session, solicit a click on an "ok" button to enter it, that button is taken to be the triggering activation)