SupervisedUserSyncService cleanup: Implement SigninManagerBase::Observer

chrome/browser/supervised_user/legacy/supervised_user_sync_service_factory.cc

 // 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.
 
 #include "chrome/browser/supervised_user/legacy/supervised_user_sync_service_factory.h"
 
 #include "chrome/browser/profiles/profile.h"
+#include "chrome/browser/signin/signin_manager_factory.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_sync_service.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 
 // static
 SupervisedUserSyncService* SupervisedUserSyncServiceFactory::GetForProfile(
     Profile* profile) {
   return static_cast<SupervisedUserSyncService*>(
       GetInstance()->GetServiceForBrowserContext(profile, true));
 }
 
 // static
 SupervisedUserSyncServiceFactory*
 SupervisedUserSyncServiceFactory::GetInstance() {
   return Singleton<SupervisedUserSyncServiceFactory>::get();
 }
 
 SupervisedUserSyncServiceFactory::SupervisedUserSyncServiceFactory()
     : BrowserContextKeyedServiceFactory(
           "SupervisedUserSyncService",
           BrowserContextDependencyManager::GetInstance()) {
+  DependsOn(SigninManagerFactory::GetInstance());
 }
 
 SupervisedUserSyncServiceFactory::~SupervisedUserSyncServiceFactory() {}
 
 KeyedService* SupervisedUserSyncServiceFactory::BuildServiceInstanceFor(
     content::BrowserContext* profile) const {
   return new SupervisedUserSyncService(
-      static_cast<Profile*>(profile)->GetPrefs());
+      static_cast<Profile*>(profile));
 }