Keep track of which attachments are referenced by which sync entries.

sync/internal_api/public/base/attachment_id_proto.cc

+// 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.
+
+#include "sync/internal_api/public/base/attachment_id_proto.h"
+
+#include "base/rand_util.h"
+
+namespace syncer {
+
+sync_pb::AttachmentIdProto CreateAttachmentIdProto() {
+  // Only requirement here is that this id must be globally unique.
+  // TODO(maniscalco): Consider making this base64 encoded.
+  sync_pb::AttachmentIdProto proto;
+  proto.set_unique_id(base::RandBytesAsString(16));
+  return proto;
+}
+
+}  // namespace syncer