Make sure WindowObservers are removed from window before destruction

ui/aura/window_observer.h

Index: ui/aura/window_observer.h
diff --git a/ui/aura/window_observer.h b/ui/aura/window_observer.h
index 5008ac514d50724cc1b924c07609e8ca015459dd..e4a4615ac30843d021ea121a209f1b9a2457046a 100644
--- a/ui/aura/window_observer.h
+++ b/ui/aura/window_observer.h
@@ -89,7 +89,9 @@ class AURA_EXPORT WindowObserver {
 
   // Invoked when the Window has been destroyed (i.e. at the end of its
   // destructor). This is called after the window is removed from its parent.
-  virtual void OnWindowDestroyed(Window* window) {}
+  // The default implementation simply remove itself from |window|'s observer

[sky, 2013/12/04 04:27:32]

If we're going to do this, then we should follow the pattern of
WebContentsObserver and have Window invoke a private function that removes this
as an observer and then invokes OnWindowDestroyed.

+  // list.
+  virtual void OnWindowDestroyed(Window* window);
 
   // Called when a Window has been added to a RootWindow.
   virtual void OnWindowAddedToRootWindow(Window* window) {}