chrome: Cleanup class/struct forward declarations

chrome/browser/prefs/pref_metrics_service.h

 // Copyright 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_PREFS_PREF_METRICS_SERVICE_H_
 #define CHROME_BROWSER_PREFS_PREF_METRICS_SERVICE_H_
 
 #include <map>
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/singleton.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/profiles/profile.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/sync_preferences/synced_pref_change_registrar.h"
 
-class PrefRegistrySimple;
-
 // PrefMetricsService is responsible for recording prefs-related UMA stats.
 class PrefMetricsService : public KeyedService {
  public:
   explicit PrefMetricsService(Profile* profile);
   ~PrefMetricsService() override;
 
   // Records metrics about the state of the homepage on launch.
   static void RecordHomePageLaunchMetrics(bool show_home_button,
                                           bool homepage_is_ntp,
                                           const GURL& homepage_url);
@@ skipping 59 matching lines @@
 
   std::unique_ptr<sync_preferences::SyncedPrefChangeRegistrar>
       synced_pref_change_registrar_;
 
   base::WeakPtrFactory<PrefMetricsService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefMetricsService);
 };
 
 #endif  // CHROME_BROWSER_PREFS_PREF_METRICS_SERVICE_H_