Drivetrain now resets the saved gyro value if it is stale.

Change-Id: I624f4c035babe8c22a633fd16dccc9b960d055c7
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index 3483b60..2b4c1f7 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -46,6 +46,8 @@
   StateFeedbackLoop<7, 2, 3> kf_;
   PolyDrivetrain dt_openloop_;
   DrivetrainMotorsSS dt_closedloop_;
+  ::aos::monotonic_clock::time_point last_gyro_time_ =
+      ::aos::monotonic_clock::min_time;
 
   StateFeedbackLoop<2, 1, 1> down_estimator_;
   Eigen::Matrix<double, 1, 1> down_U_;