chrome: Cleanup class/struct forward declarations

chrome/browser/browser_process_platform_part_mac.h

 // Copyright (c) 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.
 
 #ifndef CHROME_BROWSER_BROWSER_PROCESS_PLATFORM_PART_MAC_H_
 #define CHROME_BROWSER_BROWSER_PROCESS_PLATFORM_PART_MAC_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "chrome/browser/apps/app_shim/app_shim_host_manager_mac.h"
 #include "chrome/browser/browser_process_platform_part_base.h"
 
-namespace apps {
-class ExtensionAppShimHandler;
-}
-
 class BrowserProcessPlatformPart : public BrowserProcessPlatformPartBase {
  public:
   BrowserProcessPlatformPart();
   ~BrowserProcessPlatformPart() override;
 
   // Overridden from BrowserProcessPlatformPartBase:
   void StartTearDown() override;
   void AttemptExit() override;
   void PreMainMessageLoopRun() override;
 
   AppShimHostManager* app_shim_host_manager();
 
  private:
   // Hosts the IPC channel factory that App Shims connect to on Mac.
   scoped_refptr<AppShimHostManager> app_shim_host_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserProcessPlatformPart);
 };
 
 #endif  // CHROME_BROWSER_BROWSER_PROCESS_PLATFORM_PART_MAC_H_