According to the spec, there is no mandatory relation between segments'
sequence numbers of different variants. This CL ignores sequence numbers
when switching variants:
* In vod, the switching playback position is obtained by adding the
duration of previous segments.
* In live playback this is not possible. It is assumed that the
different live media playlists do not drift apart too much, so
the playback position is obtained by subtracting the duration
in reverse order.
In later CLs, the described mechanisms will become the fallback methods
by replacing them with the use of EXT-X-PROGRAM-DATE-TIME information
or more reliable sources.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128072663
68156ac7a4