Reads the latest client crash record for a single client.

def ReadClientCrashInfo(self, client_id):
    """Reads the latest client crash record for a single client."""
    history = self.crash_history.get(client_id, None)
    if not history:
      return None

    ts = max(history)
    res = rdf_client.ClientCrash.FromSerializedString(history[ts])
    res.timestamp = ts
    return res