FYI: Control Data + Per-Device Metadata

sync/protocol/per_user_specifics.proto

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Sync protocol datatype extension for account scope flags and data.
+
+// Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
+// any fields in this file.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+option retain_unknown_fields = true;
+
+package sync_pb;
+
+// Inforamtion about an account.
+message PerUserSpecifics {
+  // optional bool some_example_flag = 1;
+}