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

chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc

Index: chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc b/chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc
index cc191f6a22d19dfb850c2c242edb33ca61503d0d..49618f4a2b0a7278c4e52f0679b81d9ff509d77c 100644
--- a/chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc
+++ b/chrome/browser/sync_file_system/drive_backend/fake_sync_worker.cc
@@ -130,12 +130,13 @@ void FakeSyncWorker::SetSyncEnabled(bool enabled) {
     UpdateServiceState(REMOTE_SERVICE_DISABLED, "Disabled FakeSyncWorker.");
 }
 
-void FakeSyncWorker::PromoteDemotedChanges() {
+void FakeSyncWorker::PromoteDemotedChanges(const base::Closure& callback) {
   DCHECK(sequence_checker_.CalledOnValidSequencedThread());
   FOR_EACH_OBSERVER(
       Observer,
       observers_,
       OnPendingFileListUpdated(10));
+  callback.Run();
 }
 
 void FakeSyncWorker::ApplyLocalChange(