Voters in Virginia’s Washington suburbs are heading to the polls Tuesday in a special election that could offer an early glimpse of the state’s high-stakes governor’s race this fall.Republican Stewart Whitson, an Army combat veteran and former FBI agent, is challenging Democrat James Walkinshaw, a Fairfax County supervisor and longtime chief of staff to Rep. Gerry Connolly, D-Va. The Northern Virginia district typically votes overwhelmingly Democratic, giving Walkinshaw a clear edge.But analysis of recent statewide elections, some insight into the upcoming governor’s race can be gained even if Whitson does not pull off what would be a historic upset for