Store audio device's active state in preference

chromeos/audio/audio_device.h

Index: chromeos/audio/audio_device.h
diff --git a/chromeos/audio/audio_device.h b/chromeos/audio/audio_device.h
index ee47ee5e2be0b6d0c790fff388526bbc1e21b1ee..6d62a8efac7d7111f024bc6240e8c0395fcd7aab 100644
--- a/chromeos/audio/audio_device.h
+++ b/chromeos/audio/audio_device.h
@@ -57,7 +57,18 @@ struct CHROMEOS_EXPORT AudioDevice {
   }
 
   bool is_input;
+
+  // Id of this audio device. The legacy |id| is assigned to be unique everytime
+  // when each device got plugged, so that the same physical device will have
+  // a different id after unplug then re-plug.
+  // The |stable_device_id| is designed to be persistent across system reboot
+  // and plug/unplug for the same physical device. It is guaranteed that
+  // different type of hardware has different |stable_device_id|, but not
+  // guaranteed to be different between the same kind of audio device, e.g
+  // USB headset. |id| and |stable_device_id| can be used together to achieve
+  // various goals.
   uint64_t id;
+  uint64_t stable_device_id;
   std::string display_name;
   std::string device_name;
   std::string mic_positions;