[Sync] Replace AttachmentIdSet with AttachmentIdList in interfaces.

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>
@@ skipping 57 matching lines @@
 
   typedef Immutable<sync_pb::AttachmentIdProto,
                     ImmutableAttachmentIdProtoTraits>
       ImmutableAttachmentIdProto;
 
   ImmutableAttachmentIdProto proto_;
 
   AttachmentId(sync_pb::AttachmentIdProto* proto);
 };
 
+// All public interfaces use AttachmentIdList. AttachmentIdSet is used in
+// implementations of algorithms where set properties are needed.
 typedef std::vector<AttachmentId> AttachmentIdList;
 typedef std::set<AttachmentId> AttachmentIdSet;
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_