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

ash/test/test_session_state_delegate.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 "ash/test/test_session_state_delegate.h"
 
 #include <algorithm>
 #include <string>
 
 #include "ash/shell.h"
@@ skipping 81 matching lines @@
   user_list_.push_back(
       new MockUserInfo("Second@tray"));  // This is intended to be capitalized.
   user_list_.push_back(new MockUserInfo("third@tray"));
   user_list_.push_back(new MockUserInfo("someone@tray"));
 }
 
 TestSessionStateDelegate::~TestSessionStateDelegate() {
   STLDeleteElements(&user_list_);
 }
 
-void TestSessionStateDelegate::AddUser(const std::string user_id) {
+void TestSessionStateDelegate::AddUser(const std::string& user_id) {
   user_list_.push_back(new MockUserInfo(user_id));
 }
 
 const user_manager::UserInfo* TestSessionStateDelegate::GetActiveUserInfo()
     const {
   return user_list_[active_user_index_];
 }
 
 content::BrowserContext*
 TestSessionStateDelegate::GetBrowserContextByIndex(
@@ skipping 145 matching lines @@
 void TestSessionStateDelegate::AddSessionStateObserver(
     SessionStateObserver* observer) {
 }
 
 void TestSessionStateDelegate::RemoveSessionStateObserver(
     SessionStateObserver* observer) {
 }
 
 }  // namespace test
 }  // namespace ash