[Sync] Make directory types registration explicit in ModelTypeRegistry

components/sync/driver/model_association_manager.h

 // Copyright 2014 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 COMPONENTS_SYNC_DRIVER_MODEL_ASSOCIATION_MANAGER_H__
 #define COMPONENTS_SYNC_DRIVER_MODEL_ASSOCIATION_MANAGER_H__
 
 #include <map>
 
 #include "base/macros.h"
@@ skipping 10 matching lines @@
 
 // |ModelAssociationManager| does the heavy lifting for doing the actual model
 // association. It instructs DataTypeControllers to load models, start
 // associating and stopping. Since the operations are async it uses an
 // interface to inform DataTypeManager the results of the operations.
 // This class is owned by DataTypeManager.
 // |ModelAssociationManager| association functions are async. The results of
 // those operations are passed back via this interface.
 class ModelAssociationManagerDelegate {
  public:
+  // Called right before ModelAssociationManager calls LoadModels. Allows
+  // directory types to register with sync engine before download starts.
+  virtual void OnSingleDataTypeWillStart(ModelType type) = 0;
+
   // Called when all desired types are ready to be configured with
   // ModelTypeConfigurer. Data type is ready when its progress marker is
   // available to configurer. Directory data types are always ready, their
   // progress markers are read from directory. USS data type controllers need to
   // load model and read data type context first.
   // This function is called at most once after each call to
   // ModelAssociationManager::Initialize().
   virtual void OnAllDataTypesReadyForConfigure() = 0;
 
   // Called when model association (MergeDataAndStartSyncing) has completed
@@ skipping 132 matching lines @@
   bool notified_about_ready_for_configure_;
 
   base::WeakPtrFactory<ModelAssociationManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ModelAssociationManager);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_MODEL_ASSOCIATION_MANAGER_H__