Seeks are permitted when there's no timeline, and in this
case playback necessarily fails on the playback thread when
the timeline becomes known. Pushing failure to the same
place when the timeline is known means there's only one
failure path for this case. A later CL will likely introduce
an InvalidSeekPositionException for this case.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=141037708
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