Notify about major session changes events.

ash/session_state_observer.h

Index: ash/session_state_observer.h
diff --git a/ash/session_state_observer.h b/ash/session_state_observer.h
index de1b5333801e51934884c9f447da2b23db25d12c..59d6f672090339b9b843dfca94ea63c0c0ffd651 100644
--- a/ash/session_state_observer.h
+++ b/ash/session_state_observer.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "ash/ash_export.h"
+#include "ash/session_state_delegate.h"
 #include "base/basictypes.h"
 
 namespace ash {
@@ -20,6 +21,9 @@ class ASH_EXPORT SessionStateObserver {
   // Called when another user gets added to the existing session.
   virtual void UserAddedToSession(const std::string& user_id) {}
 
+  // Called when session state is changed.
+  virtual void SessionStateChanged(SessionStateDelegate::SessionState state) {}
+
  protected:
   virtual ~SessionStateObserver() {}
 };