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

components/user_manager/user_manager_base.cc

 // Copyright 2014 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 "components/user_manager/user_manager_base.h"
 
 #include <cstddef>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 772 matching lines @@
 }
 
 void UserManagerBase::SetIsCurrentUserNew(bool is_new) {
   is_current_user_new_ = is_new;
 }
 
 bool UserManagerBase::HasPendingBootstrap(const std::string& user_id) const {
   return false;
 }
 
-void UserManagerBase::SetOwnerEmail(std::string owner_user_id) {
+void UserManagerBase::SetOwnerEmail(const std::string& owner_user_id) {
   owner_email_ = owner_user_id;
 }
 
 const std::string& UserManagerBase::GetPendingUserSwitchID() const {
   return pending_user_switch_;
 }
 
-void UserManagerBase::SetPendingUserSwitchID(std::string user_id) {
+void UserManagerBase::SetPendingUserSwitchID(const std::string& user_id) {
   pending_user_switch_ = user_id;
 }
 
 void UserManagerBase::EnsureUsersLoaded() {
   DCHECK(task_runner_->RunsTasksOnCurrentThread());
   if (!GetLocalState())
     return;
 
   if (user_loading_stage_ != STAGE_NOT_LOADED)
     return;
@@ skipping 534 matching lines @@
 }
 
 void UserManagerBase::DeleteUser(User* user) {
   const bool is_active_user = (user == active_user_);
   delete user;
   if (is_active_user)
     active_user_ = NULL;
 }
 
 }  // namespace user_manager