Refactor screen/monitor so that gfx::Screen returns monitor object.

ui/aura/monitor_observer.h

+// Copyright (c) 2012 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_MONITOR_OBSERVER_H_
+#define UI_AURA_MONITOR_OBSERVER_H_
+#pragma once
+
+#include "ui/aura/aura_export.h"
+
+namespace gfx {
+class Monitor;
+}
+
+namespace aura {
+
+// Observers for monitor configuration changes.
+class AURA_EXPORT MonitorObserver {
+ public:
+  // Called when the |monitor|'s bound has changed.
+  virtual void OnMonitorBoundsChanged(const gfx::Monitor& monitor) = 0;
+
+  // Called when |new_monitor| has been added.
+  virtual void OnMonitorAdded(const gfx::Monitor& new_monitor) = 0;
+
+  // Called when |old_monitor| has been removed.
+  virtual void OnMonitorRemoved(const gfx::Monitor& old_monitor) = 0;
+
+ protected:
+  virtual ~MonitorObserver();
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MONITOR_OBSERVER_H_