SDK / exoplayer

- When calculating the downloaded percentage in DASH, there was no
  way to disambiguate between 0 of 0 segments being downloaded because
  there are no cached indexes (i.e. 0% downloaded) and 0 of 0 segments
  being downloaded because the index defines 0 segments (i.e. 100%
  downloaded).
- Also replace use of NaN with a named constant.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194453202