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

ui/gfx/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_GFX_MONITOR_OBSERVER_H_
+#define UI_GFX_MONITOR_OBSERVER_H_
+#pragma once
+
+#include "ui/base/ui_export.h"
+
+namespace gfx {
+class Monitor;
+
+// Observers for monitor configuration changes.
+class UI_EXPORT MonitorObserver {
+ public:
+  // Called when the |monitor|'s bound has changed.
+  virtual void OnMonitorBoundsChanged(const Monitor* monitor) = 0;
+
+  // Called when |new_monitor| has been added.
+  virtual void OnMonitorAdded(Monitor* new_monitor) = 0;
+
+  // Called when |old_monitor| has been removed.
+  virtual void OnMonitorRemoved(const Monitor* old_monitor) = 0;
+
+ protected:
+  MonitorObserver();
+  virtual ~MonitorObserver();
+};
+
+}  // namespace gfx
+
+#endif  // UI_GFX_MONITOR_OBSERVER_H_