Makes Panel related classes use ash/wm/common types

ash/wm/common/wm_display_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 ASH_WM_COMMON_WM_DISPLAY_OBSERVER_H_
+#define ASH_WM_COMMON_WM_DISPLAY_OBSERVER_H_
+
+#include "ash/ash_export.h"
+
+namespace ash {
+namespace wm {
+
+class WmWindow;
+
+// Used to track changes in display configuration.
+class ASH_EXPORT WmDisplayObserver {
+ public:
+  virtual void OnDisplayConfigurationChanged() {}
+
+ protected:
+  virtual ~WmDisplayObserver() {}
+};
+
+}  // namespace wm
+}  // namespace ash
+
+#endif  // ASH_WM_COMMON_WM_DISPLAY_OBSERVER_H_