Update sync API to support attachments.

sync/protocol/sync.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 for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 349 matching lines @@
   // that do not set this field.  See the comments on
   // |server_position_in_parent| for more information on how this is handled.
   //
   // This field will not be set for items whose type ignores positioning.
   // Clients should not attempt to read this field on the receipt of an item of
   // a type that ignores positioning.
   //
   // Refer to its definition in unique_position.proto for more information about
   // its internal representation.
   optional UniquePosition unique_position = 25;
+
+  // Attachment ids of attachments associated with this SyncEntity.
+  repeated AttachmentId attachment_id = 26;
 };
 
 // This message contains diagnostic information used to correlate
 // commit-related traffic with extensions-related mutations to the
 // data models in chromium.  It plays no functional role in
 // processing this CommitMessage.
 message ChromiumExtensionsActivity {
   // The human-readable ID identifying the extension responsible
   // for the traffic reported in this ChromiumExtensionsActivity.
   optional string extension_id = 1;
@@ skipping 544 matching lines @@
     // Currently only meaningful if |error_type| is throttled. If this field
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
 
   // The new per-client state for this client. If set, should be persisted and
   // sent with any subsequent ClientToServerMessages.
   optional ChipBag new_bag_of_chips = 14;
 };
-