[Refactor] Use ScopedTestPublicSessionLoginState in code

chromeos/login/scoped_test_public_session_login_state.cc

 // Copyright 2017 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/login/scoped_test_public_session_login_state.h"
 
 #include "chromeos/login/login_state.h"
 
 namespace chromeos {
 
+namespace {
+
+bool g_instance_exists = false;
+
+}  // namespace
+
 ScopedTestPublicSessionLoginState::ScopedTestPublicSessionLoginState() {
+  // Allow only one instance of this class.
+  CHECK(!g_instance_exists);
+  g_instance_exists = true;
+
   // Set Public Session state.
-  chromeos::LoginState::Initialize();
-  chromeos::LoginState::Get()->SetLoggedInState(
-      chromeos::LoginState::LOGGED_IN_ACTIVE,
-      chromeos::LoginState::LOGGED_IN_USER_PUBLIC_ACCOUNT);
+  if (!LoginState::IsInitialized()) {
+    LoginState::Initialize();
+    needs_shutdown_ = true;
+  }
+  LoginState::Get()->SetLoggedInState(
+      LoginState::LOGGED_IN_ACTIVE,
+      LoginState::LOGGED_IN_USER_PUBLIC_ACCOUNT);
 }
 
 ScopedTestPublicSessionLoginState::~ScopedTestPublicSessionLoginState() {
   // Reset state at the end of test.
-  chromeos::LoginState::Shutdown();
+  LoginState::Get()->SetLoggedInState(
+      LoginState::LOGGED_IN_NONE,
+      LoginState::LOGGED_IN_USER_NONE);
+  if (needs_shutdown_)
+    LoginState::Shutdown();
+
+  g_instance_exists = false;
 }
 
 }  // namespace chromeos