Fix bugs in the display notification

ash/system/chromeos/screen_layout_observer.h

Index: ash/system/chromeos/screen_layout_observer.h
diff --git a/ash/system/chromeos/screen_layout_observer.h b/ash/system/chromeos/screen_layout_observer.h
index 3324f7ce21858b03bd485a53990b2874f7a8a94f..eab68fe33f1030fb305ffc126af08329e11ce8bf 100644
--- a/ash/system/chromeos/screen_layout_observer.h
+++ b/ash/system/chromeos/screen_layout_observer.h
@@ -51,8 +51,8 @@ class ASH_EXPORT ScreenLayoutObserver : public WmDisplayObserver {
   // 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,
@@ -60,6 +60,16 @@ class ASH_EXPORT ScreenLayoutObserver : public WmDisplayObserver {
 
   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);