Move Singleton and related structs to namespace base

extensions/browser/api/vpn_provider/vpn_service_factory.h

Index: extensions/browser/api/vpn_provider/vpn_service_factory.h
diff --git a/extensions/browser/api/vpn_provider/vpn_service_factory.h b/extensions/browser/api/vpn_provider/vpn_service_factory.h
index 51ee00b5f34251afc43704c0a2f0d5361dd2122b..a343602ad5c83b3b78d54c0472df2fb4c8172a6c 100644
--- a/extensions/browser/api/vpn_provider/vpn_service_factory.h
+++ b/extensions/browser/api/vpn_provider/vpn_service_factory.h
@@ -14,8 +14,10 @@ class BrowserContext;
 
 }  // namespace content
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}
 
 namespace chromeos {
 
@@ -28,7 +30,7 @@ class VpnServiceFactory : public BrowserContextKeyedServiceFactory {
   static VpnServiceFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<VpnServiceFactory>;
+  friend struct base::DefaultSingletonTraits<VpnServiceFactory>;
 
   VpnServiceFactory();
   ~VpnServiceFactory() override;