Cleanup: Pass std::string as const reference if possible

chromeos/audio/audio_devices_pref_handler_impl.h

 // Copyright (c) 2013 The Chromium 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 CHROMEOS_AUDIO_AUDIO_DEVICES_PREF_HANDLER_IMPL_H_
 #define CHROMEOS_AUDIO_AUDIO_DEVICES_PREF_HANDLER_IMPL_H_
 
 #include <string>
 
 #include "base/observer_list.h"
@@ skipping 46 matching lines @@
   void UpdateDevicesVolumePref();
   void SaveDevicesVolumePref();
 
   double GetVolumeGainPrefValue(const AudioDevice& device);
   double GetDeviceDefaultOutputVolume(const AudioDevice& device);
 
   // Methods to migrate the mute and volume settings for a device from the
   // 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();
 
   scoped_ptr<base::DictionaryValue> device_mute_settings_;
   scoped_ptr<base::DictionaryValue> device_volume_settings_;
 
   PrefService* local_state_;  // not owned
 
   PrefChangeRegistrar pref_change_registrar_;
   base::ObserverList<AudioPrefObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioDevicesPrefHandlerImpl);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_AUDIO_AUDIO_DEVICES_PREF_HANDLER_IMPL_H_