chrome: Cleanup class/struct forward declarations

chrome/browser/push_messaging/push_messaging_service_impl.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 CHROME_BROWSER_PUSH_MESSAGING_PUSH_MESSAGING_SERVICE_IMPL_H_
 #define CHROME_BROWSER_PUSH_MESSAGING_PUSH_MESSAGING_SERVICE_IMPL_H_
 
 #include <stdint.h>
 #include <memory>
 #include <set>
@@ skipping 30 matching lines @@
 class ScopedKeepAlive;
 struct PushSubscriptionOptions;
 
 namespace gcm {
 class GCMDriver;
 }
 namespace instance_id {
 class InstanceIDDriver;
 }
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 class PushMessagingServiceImpl : public content::PushMessagingService,
                                  public gcm::GCMAppHandler,
                                  public content_settings::Observer,
                                  public KeyedService,
                                  public BackgroundTrigger {
  public:
   // If any Service Workers are using push, starts GCM and adds an app handler.
   static void InitializeForProfile(Profile* profile);
 
   explicit PushMessagingServiceImpl(Profile* profile);
@@ skipping 209 matching lines @@
   // we can finish processing them without being interrupted.
   std::unique_ptr<ScopedKeepAlive> in_flight_keep_alive_;
 #endif
 
   base::WeakPtrFactory<PushMessagingServiceImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PushMessagingServiceImpl);
 };
 
 #endif  // CHROME_BROWSER_PUSH_MESSAGING_PUSH_MESSAGING_SERVICE_IMPL_H_