Remove DisplayChangeObserver ash dependencies.

ash/system/chromeos/screen_layout_observer.cc

Index: ash/system/chromeos/screen_layout_observer.cc
diff --git a/ash/system/chromeos/screen_layout_observer.cc b/ash/system/chromeos/screen_layout_observer.cc
index 6d9c08e8ff7bb7db0419da127bb2a94f20f6b96f..a804840a608cd2997d88037dd5b4147f31a05400 100644
--- a/ash/system/chromeos/screen_layout_observer.cc
+++ b/ash/system/chromeos/screen_layout_observer.cc
@@ -32,6 +32,7 @@
 #include "ui/message_center/message_center.h"
 #include "ui/message_center/notification.h"
 #include "ui/message_center/notification_delegate.h"
+#include "ui/strings/grit/ui_strings.h"
 
 using message_center::Notification;
 
@@ -117,7 +118,7 @@ base::string16 GetExternalDisplayName() {
   }
 
   if (external_id == display::kInvalidDisplayId)
-    return l10n_util::GetStringUTF16(IDS_ASH_STATUS_TRAY_UNKNOWN_DISPLAY_NAME);
+    return l10n_util::GetStringUTF16(IDS_DISPLAY_NAME_UNKNOWN);
 
   // The external display name may have an annotation of "(width x height)" in
   // case that the display is rotated or its resolution is changed.