latest_rtt should be defined as a "variable of interest"

[larseggert]

latest_rtt is mentioned in the definition of time_reordering_fraction in Section 3.2.2, but isn't itself defined anywhere. However, DetectLostPackets() uses it.

My guess is that it is basically the same as the rtt_sample variable computed as a side effect in the pseudo code for OnAckReceived(). Should rtt_sample be replaced with latest_rtt here, and latest_rtt be defined as a variable?