Update sync API to support attachments.

sync/protocol/attachments.proto

+// Copyright 2014 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 for attachments.
+
+// 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;
+
+// Identifies an attachment.
+message AttachmentIdProto {
+  // Uniquely identifies the attachment. Two attachments with the same unique_id
+  // are considered equivalent.
+  optional string unique_id = 1;
+}