[Sync] Register USS types with ModelTypeRegistry before download

components/sync_driver/glue/sync_backend_registrar.h

Index: components/sync_driver/glue/sync_backend_registrar.h
diff --git a/components/sync_driver/glue/sync_backend_registrar.h b/components/sync_driver/glue/sync_backend_registrar.h
index 8e3674f3eb37cd0b9e88daafe9d7efbf8a8fc77e..d66eb5b723b11bc8b44b6cc1c9e9bc1e42df6779 100644
--- a/components/sync_driver/glue/sync_backend_registrar.h
+++ b/components/sync_driver/glue/sync_backend_registrar.h
@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include <map>
+#include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
@@ -67,6 +68,11 @@ class SyncBackendRegistrar : public syncer::SyncManager::ChangeDelegate,
   //      released.
   ~SyncBackendRegistrar() override;
 
+  // Adds |type| to set of non-blocking types. These types will have
+  // GROUP_NON_BLOCKING initial model safe group and would be treated
+  // differently in ModelTypeRegistry.
+  void RegisterNonBlockingType(syncer::ModelType type);
+
   // Informs the SyncBackendRegistrar of the currently enabled set of types.
   // These types will be placed in the passive group.  This function should be
   // called exactly once during startup.
@@ -166,6 +172,11 @@ class SyncBackendRegistrar : public syncer::SyncManager::ChangeDelegate,
   bool IsOnThreadForGroup(syncer::ModelType type,
                           syncer::ModelSafeGroup group) const;
 
+  // Returns model safe group that should be assigned to type when it is first
+  // configured (before activation). Returns GROUP_PASSIVE for directory types
+  // and GROUP_NON_BLOCKING for non-blocking types.
+  syncer::ModelSafeGroup GetInitialGroupForType(syncer::ModelType type) const;
+
   // Name used for debugging.
   const std::string name_;
 
@@ -209,6 +220,10 @@ class SyncBackendRegistrar : public syncer::SyncManager::ChangeDelegate,
   // workers, etc.
   scoped_ptr<base::Thread> sync_thread_;
 
+  // Set of types with non-blocking implementation (as opposed to directory
+  // based).
+  syncer::ModelTypeSet non_blocking_types_;
+
   DISALLOW_COPY_AND_ASSIGN(SyncBackendRegistrar);
 };