Display off is done by a callback into backlight controller

backlight_interface.h

 // Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef POWER_MANAGER_BACKLIGHT_INTERFACE_H_
 #define POWER_MANAGER_BACKLIGHT_INTERFACE_H_
 
+#include "power_manager/signal_callback.h"
+
 #include "base/basictypes.h"
 
 namespace power_manager {
 
 // Interface for getting and setting the level of the backlight.
 class BacklightInterface {
  public:
   // Set |level| to the current brightness level of the backlight, and set
   // |max| to the max brightness level of the backlight. The minimum brightness
   // level of the backlight is zero.
   //
   // On success, return true; otherwise return false.
   virtual bool GetBrightness(int64* level, int64* max) = 0;
 
   // Set |level| to the intended brightness level of the backlight.  The
   // minimum brightness level of the backlight is zero.
   //
   // On success, return true; otherwise return false.
   virtual bool GetTargetBrightness(int64* level) = 0;
 
   // Set the backlight to the specified brightness |level|.
   //
   // On success, return true; otherwise return false.
   virtual bool SetBrightness(int64 level) = 0;
 
+  // Specify a callback that will be used to determine if the screen should
+  // be turned off during a backlight transition.
+  virtual void SetScreenOffFunc(SIGNAL_CALLBACK_PTR(void, func), void *data)
+      = 0;
+
  protected:
   ~BacklightInterface() {}
 };
 
 }  // namespace power_manager
 
 #endif  // POWER_MANAGER_BACKLIGHT_INTERFACE_H_