Converts MruWindowTracker to work with common types

ash/common/wm_window_observer.h

 // Copyright 2016 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 ASH_COMMON_WM_WINDOW_OBSERVER_H_
 #define ASH_COMMON_WM_WINDOW_OBSERVER_H_
 
 #include <stdint.h>
 
 #include "ash/ash_export.h"
@@ skipping 10 matching lines @@
 class ASH_EXPORT WmWindowObserver {
  public:
   struct TreeChangeParams {
     WmWindow* target = nullptr;
     WmWindow* old_parent = nullptr;
     WmWindow* new_parent = nullptr;
   };
 
   // |window| is the window the observer was added to, which is not necessarily
   // the window that was added/removed.
+  virtual void OnWindowTreeChanging(WmWindow* window,
+                                    const TreeChangeParams& params) {}
   virtual void OnWindowTreeChanged(WmWindow* window,
                                    const TreeChangeParams& params) {}
 
   virtual void OnWindowPropertyChanged(WmWindow* window,
                                        WmWindowProperty property) {}
 
   virtual void OnWindowStackingChanged(WmWindow* window) {}
 
   virtual void OnWindowDestroying(WmWindow* window) {}
+  virtual void OnWindowDestroyed(WmWindow* window) {}
 
   virtual void OnWindowBoundsChanged(WmWindow* window,
                                      const gfx::Rect& old_bounds,
                                      const gfx::Rect& new_bounds) {}
 
   virtual void OnWindowVisibilityChanging(WmWindow* window, bool visible) {}
 
   virtual void OnWindowTitleChanged(WmWindow* window) {}
 
  protected:
   virtual ~WmWindowObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_WINDOW_OBSERVER_H_