[SyncFS] Add completion callback to PromoteDemotedChanges (1/3)

chrome/browser/sync_file_system/drive_backend/sync_engine.cc

 // Copyright 2013 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 "chrome/browser/sync_file_system/drive_backend/sync_engine.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/metrics/histogram.h"
@@ skipping 560 matching lines @@
   if (!sync_worker_)
     return;
 
   worker_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&SyncWorkerInterface::SetSyncEnabled,
                  base::Unretained(sync_worker_.get()),
                  sync_enabled));
 }
 
-void SyncEngine::PromoteDemotedChanges() {
-  if (!sync_worker_)
+void SyncEngine::PromoteDemotedChanges(const base::Closure& callback) {
+  if (!sync_worker_) {
+    callback.Run();
     return;
+  }
+
+  base::Closure relayed_callback = RelayCallbackToCurrentThread(
+      FROM_HERE, callback_tracker_.Register(callback, callback));
 
   worker_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&SyncWorkerInterface::PromoteDemotedChanges,
-                 base::Unretained(sync_worker_.get())));
+                 base::Unretained(sync_worker_.get()),
+                 relayed_callback));
 }
 
 void SyncEngine::ApplyLocalChange(
     const FileChange& local_change,
     const base::FilePath& local_path,
     const SyncFileMetadata& local_metadata,
     const fileapi::FileSystemURL& url,
     const SyncStatusCallback& callback) {
   if (GetCurrentState() == REMOTE_SERVICE_DISABLED) {
     callback.Run(SYNC_STATUS_SYNC_DISABLED);
@@ skipping 144 matching lines @@
 
 SyncStatusCallback SyncEngine::TrackCallback(
     const SyncStatusCallback& callback) {
   return callback_tracker_.Register(
       base::Bind(callback, SYNC_STATUS_ABORT),
       callback);
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system