d38c6c84a6
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