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

ui/display/screen.h

Index: ui/display/screen.h
diff --git a/ui/display/screen.h b/ui/display/screen.h
new file mode 100644
index 0000000000000000000000000000000000000000..1b8dacb691d7172d2e9045fe38c113b758bcfa63
--- /dev/null
+++ b/ui/display/screen.h
@@ -0,0 +1,24 @@
+// 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_SCREEN_H_
+#define UI_DISPLAY_SCREEN_H_
+
+#include "ui/gfx/screen.h"
+
+namespace gfx {
+class DisplayObserver;
+class Display;

[robliao, 2016/04/23 01:15:36]

Are these forward declares necessary?

[robliao, 2016/04/23 01:40:43]

This should probably be handled by screen.h based off of the below.

[oshima, 2016/04/23 04:50:03]

Done.

+}
+
+namespace display {
+
+// TODO(oshima): move the gfx::Screen to display::Screen.
+using Display = gfx::Display;
+using DisplayObserver = gfx::DisplayObserver;

[robliao, 2016/04/23 01:15:36]

The Display and DisplayObserver types should be removed.

Consumers of screen.h should directly include display.h and display_observer.h
or forward declare them.

[oshima, 2016/04/23 01:30:44]

These are because Screen has (and needs) forward decl of these classes. They do
not have to be the real consumer.
For example, consider defining a subclass of screen like this:

class MyClass : public display::Screen {
  void AddObserver(display::DisplayObserver* observer) override;
  display::Display GetPrimaryDisplay() overrride;
}

[robliao, 2016/04/23 01:40:43]

Noted. Just making sure we're not missing an opportunity to fix up files that
perform transitive includes.

+using Screen = gfx::Screen;
+
+}  // display
+
+#endif  // UI_DISPLAY_SCREEN_H_