Don't attempt to wait for session ending when it has already ended
Don't attempt to wait for session ending when it has already ended

See https://github.com/immersive-web/webxr/pull/939

KiChjang/servo,KiChjang/servo,splav/servo,DominoTree/servo,KiChjang/servo,KiChjang/servo,splav/servo,DominoTree/servo,splav/servo,DominoTree/servo,DominoTree/servo,splav/servo,KiChjang/servo,splav/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,splav/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,splav/servo