Notes:
- RollingSampleBuffer will be renamed DefaultTrackOutput in a
following CL, and variable naming will be sanitized.
- TsChunk will also be renamed to HlsMediaChunk, since it can
be used for non-TS containers (e.g. MP3).
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120240243
