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

chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc

Index: chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc b/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
index ae22a0944e65b97a454c76b2acb82decd0e8599b..cf7da82f94927cec8778b977d1472d0c966af0f9 100644
--- a/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
+++ b/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
@@ -356,8 +356,18 @@ class DriveBackendSyncTest : public testing::Test,
 
       if (local_sync_status == SYNC_STATUS_NO_CHANGE_TO_SYNC &&
           remote_sync_status == SYNC_STATUS_NO_CHANGE_TO_SYNC) {
-        remote_sync_service_->PromoteDemotedChanges();
-        local_sync_service_->PromoteDemotedChanges();
+
+        {
+          base::RunLoop run_loop;
+          remote_sync_service_->PromoteDemotedChanges(run_loop.QuitClosure());
+          run_loop.Run();
+        }
+
+        {
+          base::RunLoop run_loop;
+          local_sync_service_->PromoteDemotedChanges(run_loop.QuitClosure());
+          run_loop.Run();
+        }
 
         if (pending_remote_changes_ || pending_local_changes_)
           continue;