If MaskingMediaSource masks a multi-window media source, it may be that a period
is removed while we are using an initial unprepared masking MediaPeriod. That
means it's not guaranteed that a timeline update still contains our
unpreparedMaskingMediaPeriod and we should ignore timeline updates where the
period is no longer present because the it will be removed anyway.
PiperOrigin-RevId: 302383787
