Introduced in FlaggedRevs with Idbdab9a7396 (6bfd276e64), and I've
migrated consumers away from the singleton in I192b962147 (5ee96e5ca7)
and I04fdbd497b8 (bbdfc4c024).
The one thing that remains global by default is the context, but
VE already avoids that with setContext, and (correctly) restores this
afterward since the FlaggablePageView object is re-used by title.
That's exactly as terrible as it sounds, but at least it's a bit more
visible after my refactor.
Bug: T314008
Change-Id: I0008818ec821c35c570d9db9c82f737783e6729b