power_manager: Send BrightnessChanged signal.

powerd.h

Index: powerd.h
diff --git a/powerd.h b/powerd.h
index 62ee573d231b764f0aebb4cd6cf6cfe03d32eaa8..7424c6f37928c3480d3ff8a46923b5450652efaf 100644
--- a/powerd.h
+++ b/powerd.h
@@ -143,6 +143,10 @@ class Daemon : public XIdleMonitor {
                                     unsigned int mask,
                                     gpointer data);
 
+  // Send a D-Bus signal announcing that the screen brightness has been set to
+  // |level| (a percentage between 0 and 100).
+  void SendBrightnessChangedSignal(int level);
+
   // Generates UMA metrics on every idle event.
   void GenerateMetricsOnIdleEvent(bool is_idle, int64 idle_time_ms);