Display off is done by a callback into backlight controller

backlight.cc

 // 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.
 
 #include "power_manager/backlight.h"
 
 #include <dirent.h>
 #include <glib.h>
 #include <inttypes.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #include <string>
 
 #include "base/logging.h"
 #include "base/file_util.h"
 #include "base/string_number_conversions.h"
 #include "base/string_util.h"
 
 namespace power_manager {
 
 // Gradually change backlight level to new brightness by breaking up the
 // transition into N steps, where N = kBacklightNumSteps.
 const int kBacklightNumSteps = 8;
 // Time between backlight adjustment steps, in milliseconds.
 const int kBacklightStepTimeMs = 30;
 
+Backlight::Backlight()
+    : turn_screen_off_data_(NULL),
+      turn_screen_off_func_(NULL) {}
+
 bool Backlight::Init() {
   FilePath base_path("/sys/class/backlight");
   DIR* dir = opendir(base_path.value().c_str());
 
 #if !defined(OS_LINUX) && !defined(OS_MACOSX) && !defined(OS_FREEBSD)
   #error Port warning: depending on the definition of struct dirent, \
          additional space for pathname may be needed
 #endif
 
   if (dir) {
@@ skipping 106 matching lines @@
     int64 step_level = current_level + diff * (i + 1) / kBacklightNumSteps;
     if (step_level == previous_level)
       continue;
     g_timeout_add(i * kBacklightStepTimeMs, SetBrightnessHardThunk,
                   CreateSetBrightnessHardArgs(this, step_level, target_level));
     previous_level = step_level;
   }
   return true;
 }
 
+void Backlight::SetScreenOffFunc(SIGNAL_CALLBACK_PTR(void, func), void *data) {
+  turn_screen_off_func_ = func;
+  turn_screen_off_data_ = data;
+}
+
 bool Backlight::GetTransitionParams(int* num_steps, int* step_time_ms)
 {
   if (num_steps == NULL)
     return false;
   if (step_time_ms == NULL)
     return false;
   *num_steps = kBacklightNumSteps;
   *step_time_ms = kBacklightStepTimeMs;
   return true;
 }
 
 gboolean Backlight::SetBrightnessHard(int64 level, int64 target_level) {
   // If the target brightness of this call does not match the backlight's
   // current target brightness, it must be from an earlier backlight adjustment
   // that had a different target brightness.  In that case, it is invalidated
   // so do nothing.
   if (target_brightness_ != target_level)
     return false; // Return false so glib doesn't repeat.
   DLOG(INFO) << "Setting brightness to " << level;
   std::string buf = base::Int64ToString(level);
   bool ok = (-1 != file_util::WriteFile(brightness_path_, buf.data(),
                                         buf.size()));
   LOG_IF(WARNING, !ok) << "Can't set brightness to " << level;
+
+  if (level == 0 &&
+      target_level == 0 &&
+      turn_screen_off_func_ != NULL &&
+      turn_screen_off_data_ != NULL)
+    turn_screen_off_func_(turn_screen_off_data_);
   return false; // Return false so glib doesn't repeat.
 }
 
 }  // namespace power_manager