Replace WmWindowObserver with aura::WindowObserver.

ash/shelf/shelf_window_targeter.h

Index: ash/shelf/shelf_window_targeter.h
diff --git a/ash/shelf/shelf_window_targeter.h b/ash/shelf/shelf_window_targeter.h
index 390bd512bb5f55bb8c33ce1c7f88b4bb398aa3ce..fd14fe1934d2f2a5b5f6a46066b58a6bb3cec2bb 100644
--- a/ash/shelf/shelf_window_targeter.h
+++ b/ash/shelf/shelf_window_targeter.h
@@ -6,27 +6,28 @@
 #define ASH_SHELF_SHELF_WINDOW_TARGETER_H_
 
 #include "ash/common/shelf/wm_shelf_observer.h"
-#include "ash/common/wm_window_observer.h"
 #include "base/macros.h"
+#include "ui/aura/window_observer.h"
 #include "ui/wm/core/easy_resize_window_targeter.h"
 
 namespace ash {
 
 class WmShelf;
+class WmWindow;
 
 // ShelfWindowTargeter makes it easier to resize windows with the mouse when the
 // window-edge slightly overlaps with the shelf edge. The targeter also makes it
 // easier to drag the shelf out with touch while it is hidden.
 class ShelfWindowTargeter : public ::wm::EasyResizeWindowTargeter,
-                            public WmWindowObserver,
+                            public aura::WindowObserver,
                             public WmShelfObserver {
  public:
   ShelfWindowTargeter(WmWindow* container, WmShelf* shelf);
   ~ShelfWindowTargeter() override;
 
  private:
-  // WmWindowObserver:
-  void OnWindowDestroying(WmWindow* window) override;
+  // aura::WindowObserver:
+  void OnWindowDestroying(aura::Window* window) override;
 
   // WmShelfObserver:
   void WillChangeVisibilityState(ShelfVisibilityState new_state) override;