Implement getBestEffortTopographicallySortedNodesByRow on AbstractDirectedGraph
Summary: This is required for D9847.
Test Plan: It works.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9956