[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..10ca1dfabc56f988e03aab3a9c08fe68e65c61c9 100644
--- a/sync/internal_api/public/engine/model_safe_worker.h
+++ b/sync/internal_api/public/engine/model_safe_worker.h
@@ -42,6 +42,7 @@ 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_NON_BLOCKING,  // Models that correspond to non-blocking types.

[skym, 2016/04/01 21:58:21]

So as we migrate to USS, we'll be slowly switching everyone from their old
groups into GROUP_NON_BLOCKING?

[pavely, 2016/04/04 17:57:16]

Yes. As we switch data type controllers to be derived from
NonBlockingDataTypeController they will be put into GROUP_NON_BLOCKING group by
SyncBackendRegistrar.

   MODEL_SAFE_GROUP_COUNT,

[skym, 2016/04/01 21:58:22]

Doesn't look like this value is ever used.

[pavely, 2016/04/04 17:57:16]

Done.

 };