Moves TransientWindowClient to ui/aura/client and adds observer

ui/aura/client/transient_window_client_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 UI_AURA_CLIENT_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_
+#define UI_AURA_CLIENT_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+
+class Window;
+
+namespace client {
+
+class AURA_EXPORT TransientWindowClientObserver {
+ public:
+  // Called when a window is added as a transient child. This is called once
+  // the child is added as a transient, but before any restacking occurs.
+  virtual void OnTransientChildWindowAdded(Window* parent,
+                                           Window* transient_child) = 0;
+
+  // Called when a window is removed as a transient child. This is called once
+  // the child is removed as a transient, but before any restacking occurs.
+  virtual void OnTransientChildWindowRemoved(Window* parent,
+                                             Window* transient_child) = 0;
+
+ protected:
+  virtual ~TransientWindowClientObserver() {}
+};
+
+}  // namespace client
+}  // namespace aura
+
+#endif  // UI_AURA_CLIENT_TRANSIENT_WINDOW_CLIENT_OBSERVER_H_