[Refactor] Use ScopedTestPublicSessionLoginState in code

chromeos/login/scoped_test_public_session_login_state.h

 // 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.
 
 #ifndef CHROMEOS_LOGIN_SCOPED_TEST_PUBLIC_SESSION_LOGIN_STATE_H_
 #define CHROMEOS_LOGIN_SCOPED_TEST_PUBLIC_SESSION_LOGIN_STATE_H_
 
 #include "base/macros.h"
 
 namespace chromeos {
 
-// A class to start and shutdown public session state for a test.
+// A class to start and shutdown public session state for a test. Only one
+// instance is allowed to exist at a given time. To be instantiated on the stack
+// (so it nicely cleans up after going out of scope).
 class ScopedTestPublicSessionLoginState {
  public:
   ScopedTestPublicSessionLoginState();
   ~ScopedTestPublicSessionLoginState();
 
  private:
+  bool needs_shutdown_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(ScopedTestPublicSessionLoginState);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_SCOPED_TEST_PUBLIC_SESSION_LOGIN_STATE_H_