arc: Create sync protos for Arc package.

sync/protocol/arc_package_specifics.proto

+// Copyright 2016 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 ARC pakcages.
+
+// Update proto_{value,enum}_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;
+
+// Properties of arc package objects.
+message ArcPackageSpecifics {
+  // Unique identifier for the item:
+  // Android package name.
+  optional string package_name = 1;
+  // Android package version.
+  optional int32 package_version = 2;
+  // Last Android id generated when Android side backups data.
+  optional int64 last_backup_android_id = 3;
+  // Last time internal value that Android side backups data.
+  optional int64 last_backup_time = 4;
+}