ash: Take the 24h clock pref into consideration when showing the time.

ash/system/power/tray_power_date.h

Index: ash/system/power/tray_power_date.h
diff --git a/ash/system/power/tray_power_date.h b/ash/system/power/tray_power_date.h
index 5ca8606840bfde877db9d9a3e6fe7075fc80f1ae..9f4f327e6c5e19fe8e62506aa7e82476125867e1 100644
--- a/ash/system/power/tray_power_date.h
+++ b/ash/system/power/tray_power_date.h
@@ -6,6 +6,7 @@
 #define ASH_SYSTEM_POWER_TRAY_POWER_DATE_H_
 #pragma once
 
+#include "ash/system/power/date_format_observer.h"
 #include "ash/system/power/power_status_controller.h"
 #include "ash/system/tray/system_tray_item.h"
 
@@ -19,7 +20,8 @@ class PowerTrayView;
 }
 
 class TrayPowerDate : public SystemTrayItem,
-                      public PowerStatusController {
+                      public PowerStatusController,
+                      public DateFormatObserver {
  public:
   TrayPowerDate();
   virtual ~TrayPowerDate();
@@ -36,6 +38,9 @@ class TrayPowerDate : public SystemTrayItem,
   // Overridden from PowerStatusController.
   virtual void OnPowerStatusChanged(const PowerSupplyStatus& status) OVERRIDE;
 
+  // Overridden from DateFormatObserver.
+  virtual void OnDateFormatChanged() OVERRIDE;
+
   scoped_ptr<tray::DateView> date_;
   scoped_ptr<tray::DateView> date_tray_;