This helps to distinguish between the MediaPeriodId the deferred media period is being
created with and the MediaPeriodId which is used to create the actual underlying period.
This serves two purposes:
1. The error reporting of ad prepare errors uses the correct media period id (the
externally visible ad media period id).
2. The transition to using media period uid instead of indices is simplified.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206160463
133f5f16b8