cros: Update touchscreen status with backlights forced off state during start

ash/system/power/tablet_power_button_controller.h

Index: ash/system/power/tablet_power_button_controller.h
diff --git a/ash/system/power/tablet_power_button_controller.h b/ash/system/power/tablet_power_button_controller.h
index ef07c397eb9650042c4131787e8c9cc35fd14d19..bd7c41ce361a8422f3c607e367996286a2bdf39e 100644
--- a/ash/system/power/tablet_power_button_controller.h
+++ b/ash/system/power/tablet_power_button_controller.h
@@ -92,6 +92,10 @@ class ASH_EXPORT TabletPowerButtonController
   // Initializes |backlights_forced_off_|.
   void OnGotInitialBacklightsForcedOff(bool is_forced_off);
 
+  // Enables or disables the touchscreen, also writing its state to a pref in
+  // local state. The touchscreen is disabled when backlights are forced off.
+  void UpdateTouchscreenStatus();
+
   // Starts |shutdown_timer_| when the power button is pressed while in
   // tablet mode.
   void StartShutdownTimer();