I think that Harry will be told by Dumbledore to share things with McG through his portrait. She will take over that position for Harry, but with Dumbledore as a main advisor because I don't think he told McG everything although I think he told her a lot! He trusted her and knew she was loyal to him. He will fill her in about the horcruxes etc. Prior to his death, however, I don't believe he had told anyone but Harry about the horcruxes although it is obvious that Slughorn knewsome things....that Voldemort used them and that Dumbledore and Harry needed his memory about the discussion he had with Tom Riddle...but nothing else other than what he might have guessed.