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

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 8ac13f717ef862b8c87ebb0ccf78aa32d87e0627..2886a9628d24630831a0c381a32cadf5292bc9a4 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.cc
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.cc
@@ -234,7 +234,7 @@ void SyncWorker::SetSyncEnabled(bool enabled) {
           enabled ? "Sync is enabled" : "Sync is disabled"));
 }
 
-void SyncWorker::PromoteDemotedChanges() {
+void SyncWorker::PromoteDemotedChanges(const base::Closure& callback) {
   DCHECK(sequence_checker_.CalledOnValidSequencedThread());
 
   MetadataDatabase* metadata_db = GetMetadataDatabase();
@@ -245,6 +245,7 @@ void SyncWorker::PromoteDemotedChanges() {
         observers_,
         OnPendingFileListUpdated(metadata_db->CountDirtyTracker()));
   }
+  callback.Run();
 }
 
 void SyncWorker::ApplyLocalChange(