Move Singleton and related structs to namespace base

extensions/browser/api/dns/host_resolver_wrapper.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 "extensions/browser/api/dns/host_resolver_wrapper.h"
 
 #include "content/public/browser/resource_context.h"
 #include "net/dns/host_resolver.h"
 
 namespace extensions {
 
 HostResolverWrapper::HostResolverWrapper() : resolver_(NULL) {}
 
 // static
 HostResolverWrapper* HostResolverWrapper::GetInstance() {
-  return Singleton<extensions::HostResolverWrapper>::get();
+  return base::Singleton<extensions::HostResolverWrapper>::get();
 }
 
 net::HostResolver* HostResolverWrapper::GetHostResolver(
     content::ResourceContext* context) {
   return resolver_ ? resolver_ : context->GetHostResolver();
 }
 
 void HostResolverWrapper::SetHostResolverForTesting(
     net::HostResolver* mock_resolver) {
   resolver_ = mock_resolver;
 }
 
 }  // namespace extensions