[SyncFS] Construct and destruct sync_worker in worker_task_runner

chrome/browser/sync_file_system/drive_backend/sync_worker.cc

Index: chrome/browser/sync_file_system/drive_backend/sync_worker.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_worker.cc b/chrome/browser/sync_file_system/drive_backend/sync_worker.cc
index c2b7f87f6c4845a891b87221d8f8b1b5d3f5d626..b1c2920b433c3c3aa0c3b9308ae82c1b634f7bcf 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.cc
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.cc
@@ -85,6 +85,7 @@ void QueryAppStatusOnUIThread(
 
 }  // namespace
 
+// static
 scoped_ptr<SyncWorker> SyncWorker::CreateOnWorker(
     const base::FilePath& base_dir,
     Observer* observer,
@@ -102,13 +103,21 @@ scoped_ptr<SyncWorker> SyncWorker::CreateOnWorker(
   return sync_worker.Pass();
 }
 
+// static
+void SyncWorker::DestructOnWorker(scoped_ptr<SyncWorker> sync_worker,
+                                  scoped_ptr<Observer> worker_observer) {
+  worker_observer.reset();
+  sync_worker.reset();
+}
+
 SyncWorker::~SyncWorker() {}
 
 void SyncWorker::Initialize() {
   DCHECK(!task_manager_);
 
   task_manager_.reset(new SyncTaskManager(
-      weak_ptr_factory_.GetWeakPtr(), 0 /* maximum_background_task */));
+      weak_ptr_factory_.GetWeakPtr(),
+      0 /* maximum_background_task */));
   task_manager_->Initialize(SYNC_STATUS_OK);
 
   PostInitializeTask();
@@ -133,7 +142,6 @@ void SyncWorker::RegisterOrigin(
     return;
   }
 
-  // TODO(peria): Forward |callback| to UI thread.
   task_manager_->ScheduleSyncTask(
       FROM_HERE,
       task.PassAs<SyncTask>(),
@@ -144,7 +152,6 @@ void SyncWorker::RegisterOrigin(
 void SyncWorker::EnableOrigin(
     const GURL& origin,
     const SyncStatusCallback& callback) {
-  // TODO(peria): Forward |callback| to UI thread.
   task_manager_->ScheduleTask(
       FROM_HERE,
       base::Bind(&SyncWorker::DoEnableApp,
@@ -157,7 +164,6 @@ void SyncWorker::EnableOrigin(
 void SyncWorker::DisableOrigin(
     const GURL& origin,
     const SyncStatusCallback& callback) {
-  // TODO(peria): Forward |callback| to UI thread.
   task_manager_->ScheduleTask(
       FROM_HERE,
       base::Bind(&SyncWorker::DoDisableApp,
@@ -171,7 +177,6 @@ void SyncWorker::UninstallOrigin(
     const GURL& origin,
     RemoteFileSyncService::UninstallFlag flag,
     const SyncStatusCallback& callback) {
-  // TODO(peria): Forward |callback| to UI thread.
   task_manager_->ScheduleSyncTask(
       FROM_HERE,
       scoped_ptr<SyncTask>(