This could happen when a media source is removed from a
DynamicConcatenatingMediaSource and one of its media periods is still active.
This media period is only removed by the player after the player received
a timeline update and thus we shouldn't release the removed child source
as long as it has active media periods.
Issue:#3796
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184522836
