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..9408ee831dddb83c66c675bca8e197b96ed9b4ef 100644
--- a/ui/aura/window_observer.h
+++ b/ui/aura/window_observer.h
@@ -87,8 +87,14 @@ class AURA_EXPORT WindowObserver {
   // destructor). This is called before the window is removed from its parent.
   virtual void OnWindowDestroying(Window* window) {}
 
-  // 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.
+  // 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.  Window automatically removes its WindowObservers
+  // before calling this method, so the following code is no op.
+  //
+  // void MyWindowObserver::OnWindowDestroyed(aura::Window* window) {
+  //    window->RemoveObserver(this);
+  // }
   virtual void OnWindowDestroyed(Window* window) {}
 
   // Called when a Window has been added to a RootWindow.