This is called when theUTakeRecorderSourcesstarts a recording, after all sources have had PreRecording called on them. Implementations should avoid blocking on this call (instead place that in PreRecording) so that the sources all get StartRecording called on them as close to possible as one another. This is useful for any source that relies on platform time (or other time sources) so that a source does not spend a long time being initialized and causing different sources to record drastically different times.