Move Singleton and related structs to namespace base

ios/public/test/fake_sync_service_factory.h

Index: ios/public/test/fake_sync_service_factory.h
diff --git a/ios/public/test/fake_sync_service_factory.h b/ios/public/test/fake_sync_service_factory.h
index 543f2a2d6ea7c95bca8f6ce71cec5ba9581b8e00..ad81d8f2bf8082bb20c13d1bfb4438c8f9d23c14 100644
--- a/ios/public/test/fake_sync_service_factory.h
+++ b/ios/public/test/fake_sync_service_factory.h
@@ -9,8 +9,10 @@
 #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
 
 namespace sync_driver {
 class FakeSyncService;
@@ -34,7 +36,7 @@ class FakeSyncServiceFactory : public BrowserStateKeyedServiceFactory {
       ios::ChromeBrowserState* browser_state);
 
  private:
-  friend struct DefaultSingletonTraits<FakeSyncServiceFactory>;
+  friend struct base::DefaultSingletonTraits<FakeSyncServiceFactory>;
 
   FakeSyncServiceFactory();
   ~FakeSyncServiceFactory() override;