chromeos: Convert system tray session length limit to mojo

ash/session/session_observer.h

Index: ash/session/session_observer.h
diff --git a/ash/session/session_observer.h b/ash/session/session_observer.h
index 554273addc7a0ebcf7374277c802bd07f9b451cd..f87a0544c459d712aa8878e7287edd400b005e3e 100644
--- a/ash/session/session_observer.h
+++ b/ash/session/session_observer.h
@@ -38,6 +38,9 @@ class ASH_EXPORT SessionObserver {
   // Called when chrome is terminating.
   virtual void OnChromeTerminating() {}
 
+  // Called when the limit becomes available and when it changes.
+  virtual void OnSessionLengthLimitChanged() {}
+
  protected:
   virtual ~SessionObserver() {}
 };