Add countdown when session time is limited

ash/system/tray/system_tray_delegate.h

Index: ash/system/tray/system_tray_delegate.h
diff --git a/ash/system/tray/system_tray_delegate.h b/ash/system/tray/system_tray_delegate.h
index 30f9369da4c20f5d6595bece34e53bc01aa6bdd4..ff51bae3af19da3337b7b87778106503958fb395 100644
--- a/ash/system/tray/system_tray_delegate.h
+++ b/ash/system/tray/system_tray_delegate.h
@@ -15,6 +15,7 @@
 #include "base/i18n/time_formatting.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/string16.h"
+#include "base/time.h"
 #include "ui/gfx/image/image_skia.h"
 
 namespace ash {
@@ -299,6 +300,12 @@ class SystemTrayDelegate {
   virtual void SetVolumeControlDelegate(
       scoped_ptr<VolumeControlDelegate> delegate) = 0;
 
+  // Returns whether the session length is limited.
+  virtual bool GetSessionLengthLimited() = 0;
+
+  // Returns the remaining session time when the session length is limited.
+  virtual base::TimeDelta GetRemainingSessionTime() = 0;
+
   // Creates a dummy delegate for testing.
   static SystemTrayDelegate* CreateDummyDelegate();
 };