Move Singleton and related structs to namespace base

ios/web/net/cert_store_impl.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/web/net/cert_store_impl.h"
 
 namespace web {
 
 // static
 CertStore* CertStore::GetInstance() {
   return CertStoreImpl::GetInstance();
 }
 
 //  static
 CertStoreImpl* CertStoreImpl::GetInstance() {
-  return Singleton<CertStoreImpl>::get();
+  return base::Singleton<CertStoreImpl>::get();
 }
 
 CertStoreImpl::CertStoreImpl() {
 }
 
 CertStoreImpl::~CertStoreImpl() {
 }
 
 int CertStoreImpl::StoreCert(net::X509Certificate* cert, int group_id) {
   return store_.Store(cert, group_id);
 }
 
 bool CertStoreImpl::RetrieveCert(int cert_id,
                                  scoped_refptr<net::X509Certificate>* cert) {
   return store_.Retrieve(cert_id, cert);
 }
 
 void CertStoreImpl::RemoveCertsForGroup(int group_id) {
   store_.RemoveForRequestTracker(group_id);
 }
 
 }  // namespace web