Notify about major session changes events.

ash/shelf/shelf_layout_manager.h

Index: ash/shelf/shelf_layout_manager.h
diff --git a/ash/shelf/shelf_layout_manager.h b/ash/shelf/shelf_layout_manager.h
index d1db4d97f7d10e6fa0c3892231cc5372e63e9943..67e4201b990c7301500a0cd1053bb00800a611fc 100644
--- a/ash/shelf/shelf_layout_manager.h
+++ b/ash/shelf/shelf_layout_manager.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "ash/ash_export.h"
+#include "ash/session_state_observer.h"
 #include "ash/shelf/background_animator.h"
 #include "ash/shelf/shelf.h"
 #include "ash/shelf/shelf_types.h"
@@ -60,7 +61,8 @@ class ASH_EXPORT ShelfLayoutManager :
     public aura::client::ActivationChangeObserver,
     public DockedWindowLayoutManagerObserver,
     public keyboard::KeyboardControllerObserver,
-    public LockStateObserver {
+    public LockStateObserver,
+    public SessionStateObserver {
  public:
 
   // We reserve a small area on the edge of the workspace area to ensure that
@@ -183,6 +185,10 @@ class ASH_EXPORT ShelfLayoutManager :
   // Overridden from ash::LockStateObserver:
   virtual void OnLockStateEvent(LockStateObserver::EventType event) OVERRIDE;
 
+  // Overridden from ash::SessionStateObserver:
+  virtual void SessionStateChanged(
+      SessionStateDelegate::SessionState state) OVERRIDE;
+
   // TODO(harrym|oshima): These templates will be moved to
   // new Shelf class.
   // A helper function that provides a shortcut for choosing