libsnapshot: Re-expose InitiateMerge and ProcessUpdateState am: f6d4e74b

    // Before calling this function, all snapshots must be mapped.

    bool FinishedSnapshotWrites();

  private:

    // Initiate a merge on all snapshot devices. This should only be used after an

    // update has been marked successful after booting.

    bool InitiateMerge();

    UpdateState ProcessUpdateState(const std::function<void()>& callback = {},

                                   const std::function<bool()>& before_cancel = {});

  public:

    // Initiate the merge if necessary, then wait for the merge to finish.

    // See InitiateMerge() and ProcessUpdateState() for details.

    // Returns: