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

chromeos/audio/audio_devices_pref_handler_impl.cc

Index: chromeos/audio/audio_devices_pref_handler_impl.cc
diff --git a/chromeos/audio/audio_devices_pref_handler_impl.cc b/chromeos/audio/audio_devices_pref_handler_impl.cc
index 4e72de6c1ca6ac4bf2e597d3330e5a502170583e..17b5681ebce457ca4feffe8a9f1479fa95ee4832 100644
--- a/chromeos/audio/audio_devices_pref_handler_impl.cc
+++ b/chromeos/audio/audio_devices_pref_handler_impl.cc
@@ -187,14 +187,14 @@ void AudioDevicesPrefHandlerImpl::SaveDevicesVolumePref() {
 }
 
 void AudioDevicesPrefHandlerImpl::MigrateDeviceMuteSettings(
-    std::string active_device) {
+    const std::string& active_device) {
   int old_mute = local_state_->GetInteger(prefs::kAudioMute);
   device_mute_settings_->SetInteger(active_device, old_mute);
   SaveDevicesMutePref();
 }
 
 void AudioDevicesPrefHandlerImpl::MigrateDeviceVolumeSettings(
-    std::string active_device) {
+    const std::string& active_device) {
   double old_volume = local_state_->GetDouble(prefs::kAudioVolumePercent);
   device_volume_settings_->SetDouble(active_device, old_volume);
   SaveDevicesVolumePref();