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

sync/internal_api/public/base/attachment_id_proto.h

Index: sync/internal_api/public/base/attachment_id_proto.h
diff --git a/sync/internal_api/public/base/attachment_id_proto.h b/sync/internal_api/public/base/attachment_id_proto.h
new file mode 100644
index 0000000000000000000000000000000000000000..3332d8110366a88ee43c3612c180ca6e46be4cc2
--- /dev/null
+++ b/sync/internal_api/public/base/attachment_id_proto.h
@@ -0,0 +1,20 @@
+// 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();
+
+}  // namespace syncer
+
+#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_