ash: Cleanup class/struct forward declarations

ash/test/ash_md_test_base.h

 // 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.
 
 #ifndef ASH_TEST_ASH_MD_TEST_BASE_H_
 #define ASH_TEST_ASH_MD_TEST_BASE_H_
 
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/test/material_design_controller_test_api.h"
 #include "ash/test/ash_test_base.h"
 
 namespace ash {
 namespace test {
-class MaterialDesignControllerTestAPI;
 
 class AshMDTestBase
     : public AshTestBase,
       public testing::WithParamInterface<ash::MaterialDesignController::Mode> {
  public:
   AshMDTestBase();
   ~AshMDTestBase() override;
 
   // AshTestBase:
   void SetUp() override;
@@ skipping 10 matching lines @@
   // to adjust the display/root window size.
   int md_auto_hidden_shelf_height_offset_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(AshMDTestBase);
 };
 
 }  // namespace test
 }  // namespace ash
 
 #endif  // ASH_TEST_ASH_MD_TEST_BASE_H_