[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 {
 
 ScopedTestPublicSessionLoginState::ScopedTestPublicSessionLoginState() {
   // 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;

[Devlin, 2017/05/04 18:03:17]

It might be worth noting (in the class comments in the .h file) that this is now
designed only to nest, rather than to be serially constructed/destructed.  For
instance, consider the two patterns of calls:

State a;  // construct
State b;  // construct
~State b;  // destruct
~State a;  // destruct

This works fine, because `b` didn't initialize state and won't tear it down. 
This is also most common, since it's what's handled by stack-allocated members.

a = new State;  // construct
b = new State;  // construct
delete a;  // destruct
// Weird State
delete b;  // destruct

This is a problem, because `a` will perform the shutdown, even though `b` is
still active.

I don't think it's worth the complexity to address this, but it warrants a
comment. :)

+  }
+  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();
+  if (needs_shutdown_) {
+    LoginState::Shutdown();
+  }
 }
 
 }  // namespace chromeos