[Sync] Convert NonBlockingDataTypeController to be single-thread.

components/sync/driver/sync_client.h

Index: components/sync/driver/sync_client.h
diff --git a/components/sync/driver/sync_client.h b/components/sync/driver/sync_client.h
index 141b2a764c112e5d80fee43b22fb91f6b5ee8415..48a8611b116e488660ac3c196efa81fbd4d7bc8a 100644
--- a/components/sync/driver/sync_client.h
+++ b/components/sync/driver/sync_client.h
@@ -93,16 +93,14 @@ class SyncClient {
 
   // Returns a weak pointer to the syncable service specified by |type|.
   // Weak pointer may be unset if service is already destroyed.
-  // Note: Should only be called from the model type thread.
+  // Note: Should only be dereferenced from the model type thread.
   virtual base::WeakPtr<syncer::SyncableService> GetSyncableServiceForType(
       syncer::ModelType type) = 0;
 
-  // Returns a non-owning pointer to the service specified by |type|. Service
-  // lifetime is independent from sync thread therefore pointer should not be
-  // retained across tasks.
-  // Note: Should only be called from the model type thread.
-  // Note: should only be called by USS.
-  virtual syncer_v2::ModelTypeService* GetModelTypeServiceForType(
+  // Returns a weak pointer to the ModelTypeService specified by |type|. Weak
+  // pointer may be unset if service is already destroyed.
+  // Note: Should only be dereferenced from the model type thread.
+  virtual base::WeakPtr<syncer_v2::ModelTypeService> GetModelTypeServiceForType(
       syncer::ModelType type) = 0;
 
   // Creates and returns a new ModelSafeWorker for the group, or null if one