[Sync] Register USS types with ModelTypeRegistry before download

sync/internal_api/public/engine/model_safe_worker.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_
 #define SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 24 matching lines @@
                        // changes to these models don't need to be pushed to a
                        // native model.
   GROUP_UI,            // Models that live on UI thread and are being synced.
   GROUP_DB,            // Models that live on DB thread and are being synced.
   GROUP_FILE,          // Models that live on FILE thread and are being synced.
   GROUP_HISTORY,       // Models that live on history thread and are being
                        // synced.
   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; changes are
+                       // forwarded to these models without ModelSafeWorker/
+                       // SyncBackendRegistrar involvement.
 };
 
 SYNC_EXPORT std::string ModelSafeGroupToString(ModelSafeGroup group);
 
 // WorkerLoopDestructionObserver is notified when the thread where it works
 // is going to be destroyed.
 class WorkerLoopDestructionObserver {
  public:
   virtual void OnWorkerLoopDestroyed(ModelSafeGroup group) = 0;
 };
@@ skipping 102 matching lines @@
 SYNC_EXPORT ModelTypeSet GetRoutingInfoTypes(
     const ModelSafeRoutingInfo& routing_info);
 
 SYNC_EXPORT ModelSafeGroup GetGroupForModelType(
     const ModelType type,
     const ModelSafeRoutingInfo& routes);
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_ENGINE_MODEL_SAFE_WORKER_H_