Move Singleton and related structs to namespace base

ios/chrome/browser/history/history_service_factory.cc

 // Copyright 2015 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 "ios/chrome/browser/history/history_service_factory.h"
 
 #include "base/memory/singleton.h"
 #include "base/prefs/pref_service.h"
 #include "components/history/core/browser/history_database_params.h"
 #include "components/history/core/browser/history_service.h"
@@ skipping 34 matching lines @@
           ios::prefs::kSavingBrowserHistoryDisabled)) {
     return nullptr;
   }
 
   return static_cast<history::HistoryService*>(
       GetInstance()->GetServiceForBrowserState(browser_state, true));
 }
 
 // static
 HistoryServiceFactory* HistoryServiceFactory::GetInstance() {
-  return Singleton<HistoryServiceFactory>::get();
+  return base::Singleton<HistoryServiceFactory>::get();
 }
 
 HistoryServiceFactory::HistoryServiceFactory()
     : BrowserStateKeyedServiceFactory(
           "HistoryService",
           BrowserStateDependencyManager::GetInstance()) {
   DependsOn(ios::BookmarkModelFactory::GetInstance());
 }
 
 HistoryServiceFactory::~HistoryServiceFactory() {
@@ skipping 20 matching lines @@
 web::BrowserState* HistoryServiceFactory::GetBrowserStateToUse(
     web::BrowserState* context) const {
   return GetBrowserStateRedirectedInIncognito(context);
 }
 
 bool HistoryServiceFactory::ServiceIsNULLWhileTesting() const {
   return true;
 }
 
 }  // namespace ios