components: Cleanup class/struct fwd declarations

components/pref_registry/pref_registry_syncable.h

 // Copyright (c) 2012 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 COMPONENTS_PREF_REGISTRY_PREF_REGISTRY_SYNCABLE_H_
 #define COMPONENTS_PREF_REGISTRY_PREF_REGISTRY_SYNCABLE_H_
 
 #include <stdint.h>
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "components/prefs/pref_registry_simple.h"
 
 namespace base {
-class DictionaryValue;
-class FilePath;
-class ListValue;
 class Value;
 }
 
 // TODO(tfarina): Change this namespace to pref_registry.
 namespace user_prefs {
 
 // A PrefRegistry that forces users to choose whether each registered
 // preference is syncable or not.
 //
 // Classes or components that want to register such preferences should
@@ skipping 53 matching lines @@
                         uint32_t flags) override;
 
   SyncableRegistrationCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefRegistrySyncable);
 };
 
 }  // namespace user_prefs
 
 #endif  // COMPONENTS_PREF_REGISTRY_PREF_REGISTRY_SYNCABLE_H_