Packager uses ThreadedIO to write media segments and manifest /
playlists. There was a possibility that media segments write being
delayed and scheduled after updating manifest / playlists.
This CL fixes the race condition.
Also added a note on how segments can be synced to cloud storage to
avoid the race condition during file sync.
Also added a live WebM test.
Fixes#386.
Change-Id: Icf9c38cdec715fa3dc2836eab1511131e129fe41
KongQun Yang