Use ScopedTaskEnvironment instead of MessageLoopForUI in chromeos tests.

chromeos/cryptohome/system_salt_getter_unittest.cc

 // Copyright 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 "chromeos/cryptohome/system_salt_getter.h"
 
 #include "base/bind.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "chromeos/dbus/fake_cryptohome_client.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace chromeos {
 namespace {
 
 // Used as a GetSystemSaltCallback.
 void CopySystemSalt(std::string* out_system_salt,
                     const std::string& system_salt) {
   *out_system_salt = system_salt;
 }
 
 class SystemSaltGetterTest : public testing::Test {
  protected:
-  SystemSaltGetterTest() : fake_cryptohome_client_(NULL) {}
+  SystemSaltGetterTest()
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI) {}
 
   void SetUp() override {
     fake_cryptohome_client_ = new FakeCryptohomeClient;
     DBusThreadManager::GetSetterForTesting()->SetCryptohomeClient(
         std::unique_ptr<CryptohomeClient>(fake_cryptohome_client_));
 
     EXPECT_FALSE(SystemSaltGetter::IsInitialized());
     SystemSaltGetter::Initialize();
     ASSERT_TRUE(SystemSaltGetter::IsInitialized());
     ASSERT_TRUE(SystemSaltGetter::Get());
   }
 
   void TearDown() override {
     SystemSaltGetter::Shutdown();
     DBusThreadManager::Shutdown();
   }
 
-  base::MessageLoopForUI message_loop_;
-  FakeCryptohomeClient* fake_cryptohome_client_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
+  FakeCryptohomeClient* fake_cryptohome_client_ = nullptr;
 };
 
 TEST_F(SystemSaltGetterTest, GetSystemSalt) {
   // Try to get system salt before the service becomes available.
   fake_cryptohome_client_->SetServiceIsAvailable(false);
   std::string system_salt;
   SystemSaltGetter::Get()->GetSystemSalt(
       base::Bind(&CopySystemSalt, &system_salt));
   base::RunLoop().RunUntilIdle();
   EXPECT_TRUE(system_salt.empty());  // System salt is not returned yet.
 
   // Service becomes available.
   fake_cryptohome_client_->SetServiceIsAvailable(true);
   base::RunLoop().RunUntilIdle();
   const std::string expected_system_salt =
       SystemSaltGetter::ConvertRawSaltToHexString(
           FakeCryptohomeClient::GetStubSystemSalt());
   EXPECT_EQ(expected_system_salt, system_salt);  // System salt is returned.
 }
 
 }  // namespace
 }  // namespace chromeos