Observe for window's (self) deletion during activation/focus change

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index 9df05d953db3de68b8ee68a29ae1b5ff86d0db17..1847d25fdb753476292652162b1d3253d65ddbb3 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -428,11 +428,14 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   // Methods implementing visibility change notifications. See WindowObserver
   // for more details.
   void NotifyWindowVisibilityChanged(aura::Window* target, bool visible);
-  // Notifies this window's observers.
-  void NotifyWindowVisibilityChangedAtReceiver(aura::Window* target,
+  // Notifies this window's observers. Returns false if |this| was deleted
+  // during the call (by an observer), otherwise true.
+  bool NotifyWindowVisibilityChangedAtReceiver(aura::Window* target,
                                                bool visible);
-  // Notifies this window and its child hierarchy.
-  void NotifyWindowVisibilityChangedDown(aura::Window* target, bool visible);
+  // Notifies this window and its child hierarchy. Returns false if
+  // |this| was deleted during the call (by an observer), otherwise
+  // true.
+  bool NotifyWindowVisibilityChangedDown(aura::Window* target, bool visible);
   // Notifies this window and its parent hierarchy.
   void NotifyWindowVisibilityChangedUp(aura::Window* target, bool visible);