Revert "chromeos: Move files in //ash/common to //ash, part 3"

ash/wm_transient_window_observer.h

Index: ash/wm_transient_window_observer.h
diff --git a/ash/wm_transient_window_observer.h b/ash/wm_transient_window_observer.h
deleted file mode 100644
index 0916f4efb5e3ac78a5ed077749a6afb41882ad88..0000000000000000000000000000000000000000
--- a/ash/wm_transient_window_observer.h
+++ /dev/null
@@ -1,25 +0,0 @@
-// 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_WM_TRANSIENT_WINDOW_OBSERVER_H_
-#define ASH_WM_TRANSIENT_WINDOW_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class WmWindow;
-
-class ASH_EXPORT WmTransientWindowObserver {
- public:
-  virtual void OnTransientChildAdded(WmWindow* window, WmWindow* transient) {}
-  virtual void OnTransientChildRemoved(WmWindow* window, WmWindow* transient) {}
-
- protected:
-  virtual ~WmTransientWindowObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_WM_TRANSIENT_WINDOW_OBSERVER_H_