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

components/user_manager/user_manager_base.h

 // 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.
 
 #ifndef COMPONENTS_USER_MANAGER_USER_MANAGER_BASE_H_
 #define COMPONENTS_USER_MANAGER_USER_MANAGER_BASE_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 268 matching lines @@
   // Should be called when regular user was removed.
   virtual void OnUserRemoved(const std::string& user_id) = 0;
 
   // Getters/setters for private members.
 
   virtual void SetCurrentUserIsOwner(bool is_current_user_owner);
 
   virtual bool GetEphemeralUsersEnabled() const;
   virtual void SetEphemeralUsersEnabled(bool enabled);
 
-  virtual void SetOwnerEmail(std::string owner_user_id);
+  virtual void SetOwnerEmail(const std::string& owner_user_id);
 
   virtual const std::string& GetPendingUserSwitchID() const;
-  virtual void SetPendingUserSwitchID(std::string user_id);
+  virtual void SetPendingUserSwitchID(const std::string& user_id);
 
   // The logged-in user that is currently active in current session.
   // NULL until a user has logged in, then points to one
   // of the User instances in |users_|, the |guest_user_| instance or an
   // ephemeral user instance.
   User* active_user_;
 
   // The primary user of the current session. It is recorded for the first
   // signed-in user and does not change thereafter.
   User* primary_user_;
@@ skipping 125 matching lines @@
   scoped_refptr<base::TaskRunner> blocking_task_runner_;
 
   base::WeakPtrFactory<UserManagerBase> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UserManagerBase);
 };
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_USER_MANAGER_BASE_H_