Fix dangling observer

mash/wm/user_window_controller_impl.h

 // Copyright 2015 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 MASH_WM_USER_WINDOW_CONTROLLER_IMPL_H_
 #define MASH_WM_USER_WINDOW_CONTROLLER_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 19 matching lines @@
 
   mojom::UserWindowObserver* user_window_observer() const {
     return user_window_observer_.get();
   }
 
   void Initialize(RootWindowController* root_controller);
 
  private:
   void AssignIdIfNecessary(mus::Window* window);
 
+  // Removes observers from the window and connection.
+  void RemoveObservers(mus::Window* user_container);
+
   // Returns the window with the specified user id.
   mus::Window* GetUserWindowById(uint32_t id);
 
   // A helper to get the container for user windows.
   mus::Window* GetUserWindowContainer() const;
 
   // mus::WindowObserver:
   void OnTreeChanging(const TreeChangeParams& params) override;
+  void OnWindowDestroying(mus::Window* window) override;
 
   // mus::WindowTreeConnectionObserver:
   void OnWindowTreeFocusChanged(mus::Window* gained_focus,
                                 mus::Window* lost_focus) override;
 
   // mojom::UserWindowController:
   void AddUserWindowObserver(mojom::UserWindowObserverPtr observer) override;
   void FocusUserWindow(uint32_t window_id) override;
 
   RootWindowController* root_controller_;
   mojom::UserWindowObserverPtr user_window_observer_;
   std::unique_ptr<WindowPropertyObserver> window_property_observer_;
   uint32_t next_id_ = 1u;
 
   DISALLOW_COPY_AND_ASSIGN(UserWindowControllerImpl);
 };
 
 }  // namespace wm
 }  // namespace mash
 
 #endif  // MASH_WM_USER_WINDOW_CONTROLLER_IMPL_H_