[Sync] Register USS types with ModelTypeRegistry before download

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 ee4e9a61ca92aea26b6b3a150373d14919b67a29..a221c06d0c59c914464c5724b39bcbda9b9f9f43 100644
--- a/sync/internal_api/public/engine/model_safe_worker.h
+++ b/sync/internal_api/public/engine/model_safe_worker.h
@@ -42,7 +42,10 @@ 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.
-  MODEL_SAFE_GROUP_COUNT,
+  GROUP_NON_BLOCKING,  // Models that correspond to non-blocking types. These
+                       // models always stay in GROUP_NON_BLOCKING, changest are

[maxbogue, 2016/04/04 18:08:14]

s/, changest/; changes/

[pavely, 2016/04/05 18:21:52]

Done. Thanks.

+                       // forwarded to these models without ModelSafeWorker/
+                       // SyncBackendRegistrar involvement.
 };
 
 SYNC_EXPORT std::string ModelSafeGroupToString(ModelSafeGroup group);