chrome: Cleanup class/struct forward declarations

chrome/browser/push_messaging/push_messaging_service_impl.h

Index: chrome/browser/push_messaging/push_messaging_service_impl.h
diff --git a/chrome/browser/push_messaging/push_messaging_service_impl.h b/chrome/browser/push_messaging/push_messaging_service_impl.h
index 3e96dd98c1253ec212855cc9901574549ad2331d..59dd061e4eb00ebf2931bea6689135ec1fcb3f35 100644
--- a/chrome/browser/push_messaging/push_messaging_service_impl.h
+++ b/chrome/browser/push_messaging/push_messaging_service_impl.h
@@ -48,10 +48,6 @@ namespace instance_id {
 class InstanceIDDriver;
 }
 
-namespace user_prefs {
-class PrefRegistrySyncable;
-}
-
 class PushMessagingServiceImpl : public content::PushMessagingService,
                                  public gcm::GCMAppHandler,
                                  public content_settings::Observer,