Converts DockedWindowLayoutManager to common ash/wm types

ash/wm/common/wm_window_observer.h

Index: ash/wm/common/wm_window_observer.h
diff --git a/ash/wm/common/wm_window_observer.h b/ash/wm/common/wm_window_observer.h
index 97500c0e4bdb8a6af6bc1b36044f80b16c96fbf2..0fbf07e8f106fd6f30a2c798b27ff335b00e5b27 100644
--- a/ash/wm/common/wm_window_observer.h
+++ b/ash/wm/common/wm_window_observer.h
@@ -42,6 +42,8 @@ class ASH_EXPORT WmWindowObserver {
                                      const gfx::Rect& old_bounds,
                                      const gfx::Rect& new_bounds) {}
 
+  virtual void OnWindowVisibilityChanging(wm::WmWindow* window, bool visible) {}
+
  protected:
   virtual ~WmWindowObserver() {}
 };