The DatabaseQuotaClient::DeleteOriginData method wraps up with:

  base::Callback<void(int)> delete_callback = base::Bind(
      &DidDeleteOriginData,
      base::RetainedRef(base::ThreadTaskRunnerHandle::Get()), callback);

  PostTaskAndReplyWithResult(
      db_tracker_thread_.get(),
      FROM_HERE,
      base::Bind(&DatabaseTracker::DeleteDataForOrigin,
                 db_tracker_,
                 storage::GetIdentifierFromOrigin(origin),
                 delete_callback),
      delete_callback);

DatabaseTracker::DeleteDataForOrigin ignores the passed callback unless it returns net::ERR_IO_PENDING, in which case it will call it asynchronously via ScheduleDatabasesForDeletion

So far as I can tell, this means that the callback can be run twice - once by PostTaskAndReplyWithResult, and then again after the ScheduleDatabasesForDeletion work completes.

This is persnickety if we want to convert QuotaClient's callbacks to OnceCallbacks. It also just seems broken.