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

sync/internal_api/public/base/attachment_id_proto.cc

Index: sync/internal_api/public/base/attachment_id_proto.cc
diff --git a/sync/internal_api/public/base/attachment_id_proto.cc b/sync/internal_api/public/base/attachment_id_proto.cc
new file mode 100644
index 0000000000000000000000000000000000000000..d09c83e9567d75e33d5a422969249c499441c775
--- /dev/null
+++ b/sync/internal_api/public/base/attachment_id_proto.cc
@@ -0,0 +1,19 @@
+// 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