[USS] Change the place where SharedModelTypeProcessor got created

sync/api/model_type_service.cc

 // Copyright 2015 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.
 
 #include "sync/api/model_type_service.h"
 
+#include "sync/api/model_type_change_processor.h"
+
 namespace syncer_v2 {
 
-ModelTypeService::ModelTypeService() {}
+ModelTypeService::ModelTypeService(
+    const ChangeProcessorFactory& change_processor_factory,
+    syncer::ModelType type)
+    : change_processor_factory_(change_processor_factory), type_(type) {}
 
 ModelTypeService::~ModelTypeService() {}
 
 ModelTypeChangeProcessor* ModelTypeService::change_processor() const {
   return change_processor_.get();
 }
 
-void ModelTypeService::set_change_processor(
-    scoped_ptr<ModelTypeChangeProcessor> change_processor) {
-  DCHECK(!change_processor_);
-  change_processor_.swap(change_processor);
-  OnChangeProcessorSet();
+ModelTypeChangeProcessor* ModelTypeService::GetOrCreateChangeProcessor() {
+  if (!change_processor_) {
+    change_processor_ = change_processor_factory_.Run(type(), this);
+    DCHECK(change_processor_);
+    OnChangeProcessorSet();
+  }
+  return change_processor_.get();
 }
 
 void ModelTypeService::clear_change_processor() {
   change_processor_.reset();
 }
 
+ModelTypeChangeProcessor* ModelTypeService::OnSyncStarting(
+    const ModelTypeChangeProcessor::StartCallback& start_callback) {
+  ModelTypeChangeProcessor* processor = GetOrCreateChangeProcessor();
+  DCHECK(processor);
+  processor->OnSyncStarting(start_callback);
+  return processor;
+}
+
+syncer::ModelType ModelTypeService::type() const {
+  return type_;
+}
+
 }  // namespace syncer_v2