- Use same constant for unknown/unset microsecond times/durations.
- Change constant values to be nowhere near the "normal" range.
- Misc cleanup.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119944019
The key change here is that nextLoadPositionUs is set to -1
if we're not loading but don't have a next chunk ready to
load. This ensures that "missing chunks" in one stream don't
prevent chunks in another stream from loading. This occurs
in SmoothStreaming with TTML subtitles, where the chunks are
sparse.
