Add AttachmentMetadata to Sync's EntryKernel.

sync/protocol/attachments.proto

 // 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.
 //
 // Sync protocol for attachments.
 
 // Update proto_{value,enum}_conversions{.h,.cc,_unittest.cc} if you change any
 // fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
 // Identifies an attachment.
 message AttachmentIdProto {
   // Uniquely identifies the attachment. Two attachments with the same unique_id
   // are considered equivalent.
   optional string unique_id = 1;
 }
+
+// A collection of attachment metadata. This proto is part of EntryKernel's "on
+// disk" representation. Private to sync.
+message AttachmentMetadata {
+}