All media periods are part of a queue of windows buffered and played by
ExoPlayer. When repeating windows, the current MediaPeriodId is insufficient
to distinguish between the repetitions of the same period. This makes it hard
to see to which media period load events belong to, and it is also difficult to
determine whether two media periods belong to the same logical window or
whether they are part of different repetitions of the same window.
Therefore this change adds a unique sequence number to each window in the
sequence of windows and this sequence number is saved as part of the
MediaPeriodId.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185829509
67780bfbc2