It's no longer safe to convert END_OF_SOURCE -> duration on
the main therad, since the Timeline from which the duration
is obtained is posted to the main thread, where-as the
buffered position is passed by updating a volatile. Hence
an update to the latter might become visible to the main
thread before the corresponding Timeline.
This change moves the conversion to the playback thread.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130265893
