Publish down estimate and reset bias when enabled the first time.

Change-Id: Ibe9f276c9882083ae30dbc3b5d4bc938619a3f35
diff --git a/frc971/control_loops/drivetrain/drivetrain.q b/frc971/control_loops/drivetrain/drivetrain.q
index 4723aef..da4fadc 100644
--- a/frc971/control_loops/drivetrain/drivetrain.q
+++ b/frc971/control_loops/drivetrain/drivetrain.q
@@ -141,6 +141,9 @@
 
     // True if the output voltage was capped last cycle.
     bool output_was_capped;
+
+    // The angle of the robot relative to the ground.
+    double ground_angle;
   };
 
   queue Goal goal;