Fix bugs in the display notification

ash/system/chromeos/screen_layout_observer.h

 // Copyright (c) 2012 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 ASH_SYSTEM_CHROMEOS_SCREEN_LAYOUT_OBSERVER_H_
 #define ASH_SYSTEM_CHROMEOS_SCREEN_LAYOUT_OBSERVER_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 33 matching lines @@
   // previous data to |old_info| if it's not NULL.
   void UpdateDisplayInfo(DisplayInfoMap* old_info);
 
   // Compares the current display settings with |old_info| and determine what
   // message should be shown for notification. Returns true if there's a
   // meaningful change. Note that it's possible to return true and set
   // |message_out| to empty, which means the notification should be removed. It
   // also sets |additional_message_out| which appears in the notification with
   // the |message_out|.
   bool GetDisplayMessageForNotification(const DisplayInfoMap& old_info,
-                                        base::string16* message_out,
-                                        base::string16* additional_message_out);
+                                        base::string16* out_message,
+                                        base::string16* out_additional_message);
 
   // Creates or updates the display notification.
   void CreateOrUpdateNotification(const base::string16& message,
                                   const base::string16& additional_message);
 
   DisplayInfoMap display_info_;
 
+  enum class DisplayMode {
+    SINGLE,
+    EXTENDED,
+    MIRRORING,
+    UNIFIED,

[oshima, 2017/01/28 06:14:30]

What I was hoping for is to have explicit state including
DOCKED and for >2 case, so that we can split the logic to
detect the state, and then decide what to do based on
the state transition. Makes sense?

+  };
+
+  DisplayMode old_display_mode_ = DisplayMode::SINGLE;
+  DisplayMode current_display_mode_ = DisplayMode::SINGLE;
+
   bool show_notifications_for_testing = true;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenLayoutObserver);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_CHROMEOS_SCREEN_LAYOUT_OBSERVER_H_