Update Commit and GetUpdatesResponse messages to include attachment ids.

sync/internal_api/public/base/attachment_id_proto.h

 // 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.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_
 #define SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_
 
 #include "sync/base/sync_export.h"
 #include "sync/protocol/sync.pb.h"
 
 namespace syncer {
 
 // Helper functions that are logically part of sync_pb::AttachmentIdProto.
 
 // Creates a unique AttachmentIdProto.
 SYNC_EXPORT_PRIVATE sync_pb::AttachmentIdProto CreateAttachmentIdProto();
 
+// Creates an AttachmentMetadata object from a repeated field of
+// AttachmentIdProto objects.
+//
+// Note: each record in the AttachmentMetadata will be marked as "on the
+// server".
+SYNC_EXPORT_PRIVATE sync_pb::AttachmentMetadata CreateAttachmentMetadata(
+    const google::protobuf::RepeatedPtrField<sync_pb::AttachmentIdProto>& ids);
+
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_