This change fixes the race condition where the internal
timeline is different to the externally visible timeline
when a seek is performed. We now resolve the seek position
using the externally visible timeline, then adjust the
period index as required for the internal timeline. If the
period is missing we follow similar logic for the existing
case where the playing period is removed.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138402076
3c8db3b9cb