Add place holder to move gfx::Display/Screen to ui/display

ui/display/display_observer.h

Index: ui/display/display_observer.h
diff --git a/ui/display/display_observer.h b/ui/display/display_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..0256cd903fc915624c89248eabd487a05420c67c
--- /dev/null
+++ b/ui/display/display_observer.h
@@ -0,0 +1,17 @@
+// 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_DISPLAY_DISPLAY_OBSERVER_H_
+#define UI_DISPLAY_DISPLAY_OBSERVER_H_
+
+#include "ui/gfx/display_observer.h"
+
+namespace display {
+
+// TODO(oshima): move the gfx::DisplayObserver to display::DisplayObserver.
+using DisplayObserver = gfx::DisplayObserver;
+
+}  // display
+
+#endif  // UI_DISPLAY_DISPLAY_OBSERVER_H_