Instantiate AttachmentDownloader and use it in AttachmentServiceImpl

sync/api/attachments/attachment_id.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_API_ATTACHMENTS_ATTACHMENT_ID_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_
 
+#include <set>
 #include <string>
 #include <vector>
 
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/util/immutable.h"
 
 namespace sync_pb {
 class AttachmentIdProto;
 }  // namespace sync_pb
 
@@ skipping 40 matching lines @@
   typedef Immutable<sync_pb::AttachmentIdProto,
                     ImmutableAttachmentIdProtoTraits>
       ImmutableAttachmentIdProto;
 
   ImmutableAttachmentIdProto proto_;
 
   AttachmentId(sync_pb::AttachmentIdProto* proto);
 };
 
 typedef std::vector<AttachmentId> AttachmentIdList;
+typedef std::set<AttachmentId> AttachmentIdSet;
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_