Converts MruWindowTracker to work with common types

ash/mus/bridge/wm_window_mus.h

Index: ash/mus/bridge/wm_window_mus.h
diff --git a/ash/mus/bridge/wm_window_mus.h b/ash/mus/bridge/wm_window_mus.h
index 29ad94c13361025ed54df8faa776229605d6765b..7cea05963487789ff58407a582d809b266c1d97e 100644
--- a/ash/mus/bridge/wm_window_mus.h
+++ b/ash/mus/bridge/wm_window_mus.h
@@ -207,6 +207,7 @@ class WmWindowMus : public WmWindow, public ::mus::WindowObserver {
 
  private:
   // mus::WindowObserver:
+  void OnTreeChanging(const TreeChangeParams& params) override;
   void OnTreeChanged(const TreeChangeParams& params) override;
   void OnWindowReordered(::mus::Window* window,
                          ::mus::Window* relative_window,
@@ -220,6 +221,7 @@ class WmWindowMus : public WmWindow, public ::mus::WindowObserver {
                              const gfx::Rect& old_bounds,
                              const gfx::Rect& new_bounds) override;
   void OnWindowDestroying(::mus::Window* window) override;
+  void OnWindowDestroyed(::mus::Window* window) override;
 
   ::mus::Window* window_;