Show message to indicate that mirriring with more than 3 display is not supported.

ash/display/display_util.h

Index: ash/display/display_util.h
diff --git a/ash/display/display_util.h b/ash/display/display_util.h
index 0dfd0d4400baf52391106c3d18d6193beab73642..524f9d36bcac273fa1c8ee9b2c5ff5775b25f3e1 100644
--- a/ash/display/display_util.h
+++ b/ash/display/display_util.h
@@ -123,6 +123,11 @@ ASH_EXPORT std::string DisplayIdListToString(
 // 2) output index of id1 < output index of id2 and id2 isn't internal.
 ASH_EXPORT bool CompareDisplayIds(int64_t id1, int64_t id2);
 
+// Shows the notification message for display related issues.
+void ShowDisplayErrorNotification(int message_id);
+
+ASH_EXPORT base::string16 GetDisplayErrorNotificationMessageForTest();
+
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_UTIL_H_