[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..1a914a44c8719947d4a3db7d165d1b4792ded7d4 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);

[skym, 2016/09/12 19:57:50]

...So is there going to be an OverrideBookmarksForUssTest and an
OverrideSessionsForUssTest and etc?

[maxbogue, 2016/09/13 03:44:54]

Nope, any real datatype will have their own flag (ala DeviceInfo) to force USS
on. This one is just special because it's not a real datatype integration test,
it's just a fakey mcfake test for the sync USS infrastructure.

[skym, 2016/09/13 15:54:32]

Still completely disagree, as we discussed offline. There are two things you're
changing here, the service and the controller. Yes, you're adding a fake
service, but you're adding a real working controller. And this is the section
where you decide what controller you add. There's no fakery here.

[maxbogue, 2016/09/13 19:15:26]

Right, but it's for a fake purpose, which is why there's a magical function on
this class instead of actually having a proper Flag like any real datatype
implementation will. We won't have to add more functions like this in the
future. In fact, we'll even be able to remove this one once PREFERENCES has a
USS implementation.

+
  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_for_uss_test_;

[skym, 2016/09/12 19:57:50]

I find this variable name super misleading. Sounds like you're changing
preference values.

[maxbogue, 2016/09/13 03:44:54]

Changed to override_prefs_controller_to_uss_for_test_ as we discussed offline.

+
   DISALLOW_COPY_AND_ASSIGN(ProfileSyncComponentsFactoryImpl);
 };