Add SessionStateObserver with ActiveUserChanged()

ash/session_state_delegate.h

Index: ash/session_state_delegate.h
diff --git a/ash/session_state_delegate.h b/ash/session_state_delegate.h
index f5451cff48e1fd61d5a6297e8775521b3aaac38a..afb33f976f915515739f8094ce0500771e7f0256 100644
--- a/ash/session_state_delegate.h
+++ b/ash/session_state_delegate.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "ash/ash_export.h"
+#include "ash/session_state_observer.h"
 #include "base/string16.h"
 
 namespace gfx {
@@ -72,6 +73,10 @@ class ASH_EXPORT SessionStateDelegate {
 
   // Switches to another active user (if that user has already signed in).
   virtual void SwitchActiveUser(const std::string& email) = 0;
+
+  // Adds or removes sessions state observer.
+  virtual void AddSessionStateObserver(SessionStateObserver* observer) = 0;
+  virtual void RemoveSessionStateObserver(SessionStateObserver* observer) = 0;
 };
 
 }  // namespace ash