Cleanup: Pass std::string as const reference from chromeos/

chromeos/audio/audio_devices_pref_handler_impl.h

Index: chromeos/audio/audio_devices_pref_handler_impl.h
diff --git a/chromeos/audio/audio_devices_pref_handler_impl.h b/chromeos/audio/audio_devices_pref_handler_impl.h
index aeb9f3ef1a0936cb73bbaa6a96370848e9f9aef8..72dc335c4b5d03ce6096de3353db1730396a2fa9 100644
--- a/chromeos/audio/audio_devices_pref_handler_impl.h
+++ b/chromeos/audio/audio_devices_pref_handler_impl.h
@@ -64,8 +64,8 @@ class CHROMEOS_EXPORT AudioDevicesPrefHandlerImpl
   // previous global pref value to the new per device pref value for the
   // current active device. If a previous global setting doesn't exist, we'll
   // use default values of mute = off and volume = 75%.
-  void MigrateDeviceMuteSettings(std::string active_device);
-  void MigrateDeviceVolumeSettings(std::string active_device);
+  void MigrateDeviceMuteSettings(const std::string& active_device);
+  void MigrateDeviceVolumeSettings(const std::string& active_device);
 
   // Notifies the AudioPrefObserver for audio policy pref changes.
   void NotifyAudioPolicyChange();