Makes AshTestImpl::Create create the right impl based on config

ash/test/ash_test_impl_aura.cc

 // Copyright 2016 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 "ash/test/ash_test_impl_aura.h"
 
 #include "ash/common/test/ash_test.h"
 #include "ash/common/wm_window.h"
+#include "ash/mus/test/ash_test_impl_mus.h"
+#include "ash/public/cpp/config.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
 #include "ash/test/ash_test_base.h"
+#include "ash/test/ash_test_helper.h"
 #include "base/memory/ptr_util.h"
 #include "ui/aura/test/test_window_delegate.h"
 #include "ui/display/display_layout.h"
 #include "ui/display/manager/display_manager.h"
 #include "ui/display/screen.h"
 #include "ui/display/test/display_manager_test_api.h"
 #include "ui/wm/core/window_util.h"
 
 namespace ash {
 namespace {
@@ skipping 72 matching lines @@
   init_params->context = WmWindow::GetAuraWindow(window);
 }
 
 void AshTestImplAura::AddTransientChild(WmWindow* parent, WmWindow* window) {
   ::wm::AddTransientChild(WmWindow::GetAuraWindow(parent),
                           WmWindow::GetAuraWindow(window));
 }
 
 // static
 std::unique_ptr<AshTestImpl> AshTestImpl::Create() {
-  return base::MakeUnique<AshTestImplAura>();
+  if (test::AshTestHelper::config() == Config::CLASSIC)
+    return base::MakeUnique<AshTestImplAura>();
+
+  return base::MakeUnique<mus::AshTestImplMus>();
 }
 
 }  // namespace ash