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

chrome/browser/sync_file_system/syncable_file_system_util.cc

Index: chrome/browser/sync_file_system/syncable_file_system_util.cc
diff --git a/chrome/browser/sync_file_system/syncable_file_system_util.cc b/chrome/browser/sync_file_system/syncable_file_system_util.cc
index f649e1b0950e95d3b43ca293556d31f11e7c3a2d..e03f33986a11f6d52d8564726c5d36e686479ffc 100644
--- a/chrome/browser/sync_file_system/syncable_file_system_util.cc
+++ b/chrome/browser/sync_file_system/syncable_file_system_util.cc
@@ -38,6 +38,8 @@ const base::FilePath::CharType kSyncFileSystemDir[] =
 // Flags to enable features for testing.
 bool g_is_syncfs_v2_enabled = true;
 
+void Noop() {}
+
 }  // namespace
 
 void RegisterSyncableFileSystem() {
@@ -165,4 +167,8 @@ void RunSoon(const tracked_objects::Location& from_here,
   base::MessageLoop::current()->PostTask(from_here, callback);
 }
 
+base::Closure NoopClosure() {
+  return base::Bind(&Noop);
+}
+
 }  // namespace sync_file_system