sync: Support non-blocking initial sync in proto

sync/test/engine/fake_sync_scheduler.cc

Index: sync/test/engine/fake_sync_scheduler.cc
diff --git a/sync/test/engine/fake_sync_scheduler.cc b/sync/test/engine/fake_sync_scheduler.cc
index 9fbbf14a39cfab541f06922def827ea7be105f96..817b37c993dcc132aad5b3badbb71495eeccc4d7 100644
--- a/sync/test/engine/fake_sync_scheduler.cc
+++ b/sync/test/engine/fake_sync_scheduler.cc
@@ -40,6 +40,9 @@ void FakeSyncScheduler::ScheduleConfiguration(
   params.ready_task.Run();
 }
 
+void FakeSyncScheduler::ScheduleInitialSyncNudge(syncer::ModelType model_type) {
+}
+
 void FakeSyncScheduler::SetNotificationsEnabled(bool notifications_enabled) {
 }