Move Singleton and related structs to namespace base

ios/chrome/browser/signin/signin_error_controller_factory.h

Index: ios/chrome/browser/signin/signin_error_controller_factory.h
diff --git a/ios/chrome/browser/signin/signin_error_controller_factory.h b/ios/chrome/browser/signin/signin_error_controller_factory.h
index 04c6dc6173d80ad4347c176d2b89a1262b76372b..c0ae7191573ab0184006f70f165cb72571080590 100644
--- a/ios/chrome/browser/signin/signin_error_controller_factory.h
+++ b/ios/chrome/browser/signin/signin_error_controller_factory.h
@@ -9,8 +9,11 @@
 #include "base/memory/scoped_ptr.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
+
 class SigninErrorController;
 
 namespace ios {
@@ -26,7 +29,7 @@ class SigninErrorControllerFactory : public BrowserStateKeyedServiceFactory {
   static SigninErrorControllerFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<SigninErrorControllerFactory>;
+  friend struct base::DefaultSingletonTraits<SigninErrorControllerFactory>;
 
   SigninErrorControllerFactory();
   ~SigninErrorControllerFactory() override;