[Ozone-DRI] Add display observer

ui/ozone/platform/dri/display_change_observer.h

+// Copyright 2015 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_OZONE_PLATFORM_DRI_DISPLAY_CHANGE_OBSERVER_H_
+#define UI_OZONE_PLATFORM_DRI_DISPLAY_CHANGE_OBSERVER_H_
+
+namespace ui {
+
+class HardwareDisplayController;
+
+class DisplayChangeObserver {
+ public:
+  virtual ~DisplayChangeObserver() {}
+
+  // Called when |controller| was changed/added.
+  virtual void OnDisplayChanged(HardwareDisplayController* controller) = 0;
+
+  // Called just before |controller| is removed. Observers that cached
+  // |controller| must invalidate it at this point.
+  virtual void OnDisplayRemoved(HardwareDisplayController* controller) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_OZONE_PLATFORM_DRI_DISPLAY_CHANGE_OBSERVER_H_