[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..864e98c7b995b5c6247e938b7b16d7b36371ba6a 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
@@ -61,6 +61,8 @@ class SyncWorker : public SyncTaskManager::Client {
 
   class Observer {
    public:
+    virtual ~Observer() {}
+
     virtual void OnPendingFileListUpdated(int item_count) = 0;
     virtual void OnFileStatusChanged(const fileapi::FileSystemURL& url,
                                      SyncFileStatus file_status,
@@ -68,9 +70,6 @@ class SyncWorker : public SyncTaskManager::Client {
                                      SyncDirection direction) = 0;
     virtual void UpdateServiceState(RemoteServiceState state,
                                     const std::string& description) = 0;
-
-   protected:
-    virtual ~Observer() {}
   };
 
   static scoped_ptr<SyncWorker> CreateOnWorker(
@@ -80,6 +79,9 @@ class SyncWorker : public SyncTaskManager::Client {
       scoped_ptr<SyncEngineContext> sync_engine_context,
       leveldb::Env* env_override);
 
+  static void DestructOnWorker(scoped_ptr<SyncWorker> sync_worker,
+                               scoped_ptr<Observer> worker_observer);
+
   virtual ~SyncWorker();
 
   void Initialize();