Use ScopedTaskEnvironment instead of MessageLoopForUI in ui/aura/test/.

ui/views/test/views_test_helper_aura.h

 // Copyright 2014 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 UI_VIEWS_TEST_VIEWS_TEST_HELPER_AURA_H_
 #define UI_VIEWS_TEST_VIEWS_TEST_HELPER_AURA_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/views/test/views_test_helper.h"
 
 namespace aura {
 class WindowTreeClient;
 
 namespace client {
 class ScreenPositionClient;
 }
 namespace test {
 class AuraTestHelper;
 }
 }
 
-namespace base {
-class MessageLoopForUI;
-}
-
 namespace views {
 
 class ViewsTestHelperAura : public ViewsTestHelper {
  public:
-  ViewsTestHelperAura(base::MessageLoopForUI* message_loop,
-                      ui::ContextFactory* context_factory,
+  ViewsTestHelperAura(ui::ContextFactory* context_factory,
                       ui::ContextFactoryPrivate* context_factory_private);
   ~ViewsTestHelperAura() override;
 
   void EnableMusWithWindowTreeClient(
       aura::WindowTreeClient* window_tree_client);
 
   // Overridden from ViewsTestHelper:
   void SetUp() override;
   void TearDown() override;
   gfx::NativeWindow GetContext() override;
 
  private:
   ui::ContextFactory* context_factory_;
   ui::ContextFactoryPrivate* context_factory_private_;
   std::unique_ptr<aura::test::AuraTestHelper> aura_test_helper_;
   std::unique_ptr<aura::client::ScreenPositionClient> screen_position_client_;
 
   DISALLOW_COPY_AND_ASSIGN(ViewsTestHelperAura);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_TEST_VIEWS_TEST_HELPER_AURA_H_