sync: Process 'control' data separately

sync/internal_api/public/engine/model_safe_worker.h

Index: sync/internal_api/public/engine/model_safe_worker.h
diff --git a/sync/internal_api/public/engine/model_safe_worker.h b/sync/internal_api/public/engine/model_safe_worker.h
index e1d6563d14ea88ff9e7abb09dd768469a4b63261..c3f2a8cd1d961fe0ecebde0b2f1b483ca9edc3e6 100644
--- a/sync/internal_api/public/engine/model_safe_worker.h
+++ b/sync/internal_api/public/engine/model_safe_worker.h
@@ -35,6 +35,8 @@ enum ModelSafeGroup {
   GROUP_PASSWORD,      // Models that live on the password thread and are
                        // being synced.  On windows and linux, this runs on the
                        // DB thread.
+  GROUP_CONTROL,      // Models that require special handling on the sync
+                       // thread.  These types have no associated model.
   MODEL_SAFE_GROUP_COUNT,
 };