[Sync] Add a sanity integration test for USS.

components/browser_sync/browser/profile_sync_components_factory_impl.h

Index: components/browser_sync/browser/profile_sync_components_factory_impl.h
diff --git a/components/browser_sync/browser/profile_sync_components_factory_impl.h b/components/browser_sync/browser/profile_sync_components_factory_impl.h
index 6b3024b04ba1605e07fe72235081b6ddfd8dcd6f..8162edbed3746f0109ebe2b414c02aee148d3d36 100644
--- a/components/browser_sync/browser/profile_sync_components_factory_impl.h
+++ b/components/browser_sync/browser/profile_sync_components_factory_impl.h
@@ -88,6 +88,10 @@ class ProfileSyncComponentsFactoryImpl
       sync_driver::SyncService* sync_service,
       std::unique_ptr<syncer::DataTypeErrorHandler> error_handler) override;
 
+  // Sets a bit that determines whether PREFERENCES should be registered with a
+  // ModelTypeController for testing purposes.
+  static void OverridePrefsForUssTest(bool use_uss);
+
  private:
   // Register data types which are enabled on both desktop and mobile.
   // |disabled_types| and |enabled_types| correspond only to those types
@@ -117,6 +121,9 @@ class ProfileSyncComponentsFactoryImpl
 
   base::WeakPtrFactory<ProfileSyncComponentsFactoryImpl> weak_factory_;
 
+  // Whether to override PREFERENCES to use USS.
+  static bool override_prefs_controller_to_uss_for_test_;
+
   DISALLOW_COPY_AND_ASSIGN(ProfileSyncComponentsFactoryImpl);
 };