[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/internal_api/public/shared_model_type_processor.h"
+
 namespace syncer_v2 {
 
-ModelTypeService::ModelTypeService() {}
+ModelTypeService::ModelTypeService(
+    const SharedProcessorFactory& shared_processor_factory)
+    : shared_processor_factory_(shared_processor_factory) {}
 
 ModelTypeService::~ModelTypeService() {}
 
-ModelTypeChangeProcessor* ModelTypeService::change_processor() const {
+ModelTypeChangeProcessor* ModelTypeService::change_processor() {
   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::create_change_processor() {
+  if (!change_processor_.get()) {
+    change_processor_.reset(
+        shared_processor_factory_.Run(type(), this).release());
+    if (!change_processor_.get()) {
+      LOG(WARNING) << "Cannot initalize ModelTypeChangeProcessor.";
+    }
+    OnChangeProcessorSet();
+  }
+  return change_processor_.get();
 }
 
 void ModelTypeService::clear_change_processor() {
   change_processor_.reset();
 }
 
+void ModelTypeService::OnSyncStarting(StartCallback start_callback) {
+  // TODO(gangwu): should not cast here, find a better way to call.

[skym, 2016/03/22 15:56:26]

Ooooh yeah I didn't realize we were going to have to cast for this to work.
Hmm.. Does it make sense to move this method to the left hand side processor
interface?

[Gang Wu, 2016/03/22 17:47:51]

put this method into ModelTypeChangeProcessor is weird as well, since
ModelTypeChangeProcessor is to inform sync of local changes, and the callback is
to tell controller/service that processor is started.

+  static_cast<SharedModelTypeProcessor*>(create_change_processor())
+      ->OnSyncStarting(start_callback);
+}
+
 }  // namespace syncer_v2