components: Cleanup class/struct fwd declarations

components/data_reduction_proxy/core/browser/data_reduction_proxy_prefs.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 COMPONENTS_DATA_REDUCTION_PROXY_CORE_BROWSER_DATA_REDUCTION_PROXY_PREFS_H_
 #define COMPONENTS_DATA_REDUCTION_PROXY_CORE_BROWSER_DATA_REDUCTION_PROXY_PREFS_H_
 
 #include "base/macros.h"
 
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
 class PrefRegistrySimple;
-class PrefService;
 
 namespace data_reduction_proxy {
 
 // Registers the data reduction proxy's profile prefs on platforms that use
 // syncable prefs.
 void RegisterSyncableProfilePrefs(
     user_prefs::PrefRegistrySyncable* registry);
 
 // Registers the data reduction proxy's profile prefs on platforms that do not
 // use syncable prefs.
 void RegisterSimpleProfilePrefs(PrefRegistrySimple* registry);
 
 // Registers local state, i.e., profile-agnostic prefs for the data
 // reduction proxy.
 void RegisterPrefs(PrefRegistrySimple* registry);
 
 }  // namespace data_reduction_proxy
 
 #endif  // COMPONENTS_DATA_REDUCTION_PROXY_CORE_BROWSER_DATA_REDUCTION_PROXY_PREFS_H_