extensions: Cleanup class/struct fwd declarations

extensions/browser/extension_system.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 EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_
 #define EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "build/build_config.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "extensions/common/extension.h"
 #include "extensions/features/features.h"
 
 #if !BUILDFLAG(ENABLE_EXTENSIONS)
 #error "Extensions must be enabled"
 #endif
 
 class ExtensionService;
 
-#if defined(OS_CHROMEOS)
-namespace chromeos {
-class DeviceLocalAccountManagementPolicyProvider;
-}
-#endif  // defined(OS_CHROMEOS)
-
 namespace content {
 class BrowserContext;
 }
 
 namespace extensions {
 
 class AppSorting;
 class ContentVerifier;
 class Extension;
 class ExtensionSet;
@@ skipping 94 matching lines @@
   // temp_dir. Ownership of |temp_dir| in the filesystem is transferred and
   // implementors of this function are responsible for cleaning it up on
   // errors, etc.
   virtual void InstallUpdate(const std::string& extension_id,
                              const base::FilePath& temp_dir) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_