The DisplayController interface in services/ui/public/interfaces/display/display_controller.mojom needs to expose several power-related methods from display::DisplayConfigurator:

  (this enum comes from dbus/service_constants.h in the shared system_api repo)
  enum DisplayPowerState {
    DISPLAY_POWER_ALL_ON = 0,
    DISPLAY_POWER_ALL_OFF = 1,
    DISPLAY_POWER_INTERNAL_OFF_EXTERNAL_ON = 2,
    DISPLAY_POWER_INTERNAL_ON_EXTERNAL_OFF = 3,
  };

  // Flags that can be passed to SetDisplayPower().
  static const int kSetDisplayPowerNoFlags;
  // Configure displays even if the passed-in state matches |power_state_|.
  static const int kSetDisplayPowerForceProbe;
  // Do not change the state if multiple displays are connected or if the
  // only connected display is external.
  static const int kSetDisplayPowerOnlyIfSingleInternalDisplay;

  // Called when powerd notifies us that some set of displays should be turned
  // on or off.  This requires enabling or disabling the CRTC associated with
  // the display(s) in question so that the low power state is engaged.
  // |flags| contains bitwise-or-ed kSetDisplayPower* values. After the
  // configuration finishes |callback| is called with the status of the
  // operation.
  void SetDisplayPower(chromeos::DisplayPowerState power_state,
                       int flags,
                       const ConfigurationCallback& callback);

  // Sets all the displays into pre-suspend mode; usually this means
  // configure them for their resume state. This allows faster resume on
  // machines where display configuration is slow. On completion of the display
  // configuration |callback| is executed synchronously or asynchronously.
  void SuspendDisplays(const ConfigurationCallback& callback);
  
  // Reprobes displays to handle changes made while the system was
  // suspended.
  void ResumeDisplays();

SetDisplayPower is used to service D-Bus method calls from powerd, while SuspendDisplays and ResumeDisplays are called within ash when the system is about to suspend or has just resumed. The callbacks may make these tricky.

I'm commenting out ash's SuspendDisplays and ResumeDisplays calls for now to avoid the crash in  issue 686938 , but it should be updated to call into mus when possible.