Move Screen, its impls, DisplayObserver and DisplayChangeNotifier to ui/display

ash/system/web_notification/ash_popup_alignment_delegate.h

Index: ash/system/web_notification/ash_popup_alignment_delegate.h
diff --git a/ash/system/web_notification/ash_popup_alignment_delegate.h b/ash/system/web_notification/ash_popup_alignment_delegate.h
index 6917db9e6c32676b0fe7134d9577077352c8db6d..50b70621a5748d17eea727bdbdb7b0e911017fec 100644
--- a/ash/system/web_notification/ash_popup_alignment_delegate.h
+++ b/ash/system/web_notification/ash_popup_alignment_delegate.h
@@ -20,12 +20,8 @@ namespace aura {
 class Window;
 }
 
-namespace gfx {
-class Screen;
-}
-
 namespace display {
-using Screen = gfx::Screen;
+class Screen;
 }
 
 namespace ash {