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

chromeos/audio/audio_devices_pref_handler_impl.cc

 // 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.
 
 #include "chromeos/audio/audio_devices_pref_handler_impl.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 169 matching lines @@
   while (!it.IsAtEnd()) {
     double volume = kDefaultOutputVolumePercent;
     bool success = it.value().GetAsDouble(&volume);
     DCHECK(success);
     dict_update->SetDouble(it.key(), volume);
     it.Advance();
   }
 }
 
 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();
 }
 
 void AudioDevicesPrefHandlerImpl::NotifyAudioPolicyChange() {
   FOR_EACH_OBSERVER(AudioPrefObserver,
                     observers_,
                     OnAudioPolicyPrefChanged());
 }
 
 // static
 void AudioDevicesPrefHandlerImpl::RegisterPrefs(PrefRegistrySimple* registry) {
   registry->RegisterDictionaryPref(prefs::kAudioDevicesVolumePercent);
   registry->RegisterDictionaryPref(prefs::kAudioDevicesMute);
 
   // Register the prefs backing the audio muting policies.
   // Policy for audio input is handled by kAudioCaptureAllowed in the Chrome
   // media system.
   registry->RegisterBooleanPref(prefs::kAudioOutputAllowed, true);
 
   // Register the legacy audio prefs for migration.
   registry->RegisterDoublePref(prefs::kAudioVolumePercent,
                                kDefaultOutputVolumePercent);
   registry->RegisterIntegerPref(prefs::kAudioMute, kPrefMuteOff);
 }
 
 }  // namespace chromeos