(Bug 50408) Make migrateStartMetas more robust

* While the bug in this case is from async pipeline issues,
  this patch adds checks to prevent crashers in migrateStartMetas
  by refusing to attempt a migration of the meta out of the
  document iself!

Change-Id: I941cc217d2f0ea693434e13364f7fdc88b46172e
Merge "(Bug 50408) Make migrateStartMetas more robust"