WASHINGTON — John Roberts is heading a Supreme Court in crisis.The chief justice has already ordered an investigation of the leak this week of a draft opinion suggesting the court could be poised to overturn Roe v. Wade, the 1973 case legalizing abortion nationwide.What comes next could further test Roberts’ leadership of a court where his vote already appears less crucial in determining the outcome in contentious cases.“This is a time when the court is under siege, both externally and internally now …