When this API was implemented, we had to use the
index of a change message because at that time we had
change messages from both ReviewDb and NoteDb but with
different IDs. Thus we made the decision to use the
"index" of a change message as the bridge between NoteDb
and ReviewDb. IOW, we assumed that a change message with
"index = k" corresponds to the kth change meta commit.
However, this is not true because empty change messages
are skipped from the change message list in
ChangeNotesParser#parseChangeMessage. When a change
has empty change message, DeleteChangeMessage could
delete the wrong message.
Now, we don't have ReviewDb any more. It makes sense
to use the ID of the change message directly and thus
fix the above issue.
Change-Id: I09b0e16d9e014f674a7bee5e798eee8f635b6bdd
8c1ebed5ed