Polish launcher tooltip visibility (2nd try).

ash/wm/shelf_layout_manager.h

Index: ash/wm/shelf_layout_manager.h
diff --git a/ash/wm/shelf_layout_manager.h b/ash/wm/shelf_layout_manager.h
index dac537abc70a67899629c6dac213aa1e674ab98b..2a4412c033ac61973d2007353e824c82247106c3 100644
--- a/ash/wm/shelf_layout_manager.h
+++ b/ash/wm/shelf_layout_manager.h
@@ -12,6 +12,7 @@
 #include "ash/wm/shelf_types.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/observer_list.h"
 #include "base/timer.h"
 #include "ui/aura/client/activation_change_observer.h"
 #include "ui/aura/layout_manager.h"
@@ -59,6 +60,18 @@ class ASH_EXPORT ShelfLayoutManager :
     AUTO_HIDE_HIDDEN,
   };
 
+  class ASH_EXPORT Observer {
+   public:
+    // Called when the target ShelfLayoutManager will be deleted.
+    virtual void WillDeleteShelf() {}
+
+    // Called when the visibility change is scheduled.
+    virtual void WillChangeVisibilityState(VisibilityState new_state) {}
+
+    // Called when the auto hide state is changed.
+    virtual void OnAutoHideStateChanged(AutoHideState new_state) {}
+  };
+
   // We reserve a small area at the bottom of the workspace area to ensure that
   // the bottom-of-window resize handle can be hit.
   // TODO(jamescook): Some day we may want the workspace area to be an even
@@ -130,6 +143,9 @@ class ASH_EXPORT ShelfLayoutManager :
   // the shelf renders slightly differently.
   void SetWindowOverlapsShelf(bool value);
 
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
   // Overridden from aura::LayoutManager:
   virtual void OnWindowResized() OVERRIDE;
   virtual void OnWindowAddedToLayout(aura::Window* child) OVERRIDE;
@@ -252,6 +268,8 @@ class ASH_EXPORT ShelfLayoutManager :
   // trigger showing the launcher.
   scoped_ptr<AutoHideEventFilter> event_filter_;
 
+  ObserverList<Observer> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(ShelfLayoutManager);
 };