[SyncFS] Construct and destruct sync_worker in worker_task_runner

chrome/browser/sync_file_system/drive_backend/sync_worker.h

Index: chrome/browser/sync_file_system/drive_backend/sync_worker.h
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_worker.h b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
index 36742c77a6291d561df0cec7dc0d442ffad7ca4a..933c2c41cb50075d7387db51eb1a2d3e0d62f869 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
@@ -73,12 +73,10 @@ class SyncWorker : public SyncTaskManager::Client {
     virtual ~Observer() {}
   };
 
-  static scoped_ptr<SyncWorker> CreateOnWorker(
-      const base::FilePath& base_dir,
-      Observer* observer,
-      const base::WeakPtr<ExtensionServiceInterface>& extension_service,
-      scoped_ptr<SyncEngineContext> sync_engine_context,
-      leveldb::Env* env_override);
+  SyncWorker(const base::FilePath& base_dir,
+             const base::WeakPtr<ExtensionServiceInterface>& extension_service,
+             scoped_ptr<SyncEngineContext> sync_engine_context,
+             leveldb::Env* env_override);
 
   virtual ~SyncWorker();
 
@@ -140,11 +138,6 @@ class SyncWorker : public SyncTaskManager::Client {
   friend class DriveBackendSyncTest;
   friend class SyncEngineTest;
 
-  SyncWorker(const base::FilePath& base_dir,
-             const base::WeakPtr<ExtensionServiceInterface>& extension_service,
-             scoped_ptr<SyncEngineContext> sync_engine_context,
-             leveldb::Env* env_override);
-
   void DoDisableApp(const std::string& app_id,
                     const SyncStatusCallback& callback);
   void DoEnableApp(const std::string& app_id,