Display off is done by a callback into backlight controller

backlight.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_H_
 #define POWER_MANAGER_BACKLIGHT_H_
 
 #include "base/basictypes.h"
 #include "base/file_path.h"
 #include "power_manager/backlight_interface.h"
@@ skipping 10 matching lines @@
 //   int64 level, max;
 //   if (backlight.Init() && backlight.GetBrightness(&level, &max)) {
 //     std::cout << "Current brightness level is "
 //               << level << " out of " << max << "\n";
 //   } else {
 //     std::cout << "Cannot get brightness level\n";
 //   }
 
 class Backlight : public BacklightInterface {
  public:
-  Backlight() {}
+  Backlight();
   virtual ~Backlight() {}
 
   // Initialize the backlight object.
   //
   // On success, return true; otherwise return false.
   bool Init();
 
   // Overridden from BacklightInterface:
   virtual bool GetBrightness(int64* level, int64* max);
   virtual bool GetTargetBrightness(int64* level);
   virtual bool SetBrightness(int64 level);
+  virtual void SetScreenOffFunc(SIGNAL_CALLBACK_PTR(void, func), void *data);
 
   // Returns info about the gradual brightness transition, represented as the
   // number of steps and the interval of each step in milliseconds.
   bool GetTransitionParams(int* num_steps, int* step_time_ms);
 
  private:
   // Look for the existence of required files and return the granularity of
   // the given backlight interface directory path.
   int64 CheckBacklightFiles(const FilePath& dir_path);
 
@@ skipping 16 matching lines @@
   SIGNAL_CALLBACK_2(Backlight, gboolean, SetBrightnessHard, int64, int64);
 
   // Paths to the actual_brightness, brightness, and max_brightness files
   // under /sys/class/backlight.
   FilePath actual_brightness_path_;
   FilePath brightness_path_;
   FilePath max_brightness_path_;
 
   int64 target_brightness_;  // The current intended brightness level.
 
+  // Callback function and data to determine whether to turn screen off.
+  void *turn_screen_off_data_;
+  SIGNAL_CALLBACK_PTR(void, turn_screen_off_func_);
+
   DISALLOW_COPY_AND_ASSIGN(Backlight);
 };
 
 }  // namespace power_manager
 
 #endif  // POWER_MANAGER_BACKLIGHT_H_