aura: Reland r266681 to debug WindowObserver crashes.

ui/aura/window_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_WINDOW_OBSERVER_H_
 #define UI_AURA_WINDOW_OBSERVER_H_
 
 #include "base/basictypes.h"
 #include "ui/aura/aura_export.h"
 
@@ skipping 13 matching lines @@
       HIERARCHY_CHANGED
     };
 
     Window* target;     // The window that was added or removed.
     Window* new_parent;
     Window* old_parent;
     HierarchyChangePhase phase;
     Window* receiver;   // The window receiving the notification.
   };
 
+  WindowObserver();
+
   // Called when a window is added or removed. Notifications are sent to the
   // following hierarchies in this order:
   // 1. |target|.
   // 2. |target|'s child hierarchy.
   // 3. |target|'s parent hierarchy in its |old_parent|
   //        (only for Changing notifications).
   // 3. |target|'s parent hierarchy in its |new_parent|.
   //        (only for Changed notifications).
   // This sequence is performed via the Changing and Changed notifications below
   // before and after the change is committed.
@@ skipping 60 matching lines @@
   // Called when a Window has been added to a RootWindow.
   virtual void OnWindowAddedToRootWindow(Window* window) {}
 
   // Called when a Window is about to be removed from a root Window.
   // |new_root| contains the new root Window if it is being added to one
   // atomically.
   virtual void OnWindowRemovingFromRootWindow(Window* window,
                                               Window* new_root) {}
 
  protected:
-  virtual ~WindowObserver() {}
+  virtual ~WindowObserver();
+
+ private:
+  friend class Window;
+
+  // Called when this is added as an observer on |window|.
+  void OnObservingWindow(Window* window);
+
+  // Called when this is removed from the observers on |window|.
+  void OnUnobservingWindow(Window* window);
+
+  // Tracks the number of windows being observed to track down
+  // http://crbug.com/365364.
+  int observing_;
+
+  DISALLOW_COPY_AND_ASSIGN(WindowObserver);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_OBSERVER_H_