Move low battery notification to Message Center.

ash/system/chromeos/power/tray_power.h

Index: ash/system/chromeos/power/tray_power.h
diff --git a/ash/system/chromeos/power/tray_power.h b/ash/system/chromeos/power/tray_power.h
index c8ae131ebd4f90f45c7007dcdb67d043a31ec374..bb1b53037abafe292885ed12d137cf2dcd6d694a 100644
--- a/ash/system/chromeos/power/tray_power.h
+++ b/ash/system/chromeos/power/tray_power.h
@@ -20,15 +20,16 @@ class MessageCenter;
 }
 
 namespace ash {
+
+class BatteryNotification;
+
 namespace tray {
-class PowerNotificationView;
 class PowerTrayView;
 }
 
 class ASH_EXPORT TrayPower : public SystemTrayItem,
                              public PowerStatus::Observer {
  public:
-  // Visible for testing.
   enum NotificationState {
     NOTIFICATION_NONE,
 
@@ -70,10 +71,8 @@ class ASH_EXPORT TrayPower : public SystemTrayItem,
   // Overridden from SystemTrayItem.
   views::View* CreateTrayView(user::LoginStatus status) override;
   views::View* CreateDefaultView(user::LoginStatus status) override;
-  views::View* CreateNotificationView(user::LoginStatus status) override;
   void DestroyTrayView() override;
   void DestroyDefaultView() override;
-  void DestroyNotificationView() override;
   void UpdateAfterLoginStatusChange(user::LoginStatus status) override;
   void UpdateAfterShelfAlignmentChange(ShelfAlignment alignment) override;
 
@@ -91,7 +90,7 @@ class ASH_EXPORT TrayPower : public SystemTrayItem,
 
   message_center::MessageCenter* message_center_;  // Not owned.
   tray::PowerTrayView* power_tray_;
-  tray::PowerNotificationView* notification_view_;
+  scoped_ptr<BatteryNotification> battery_notification_;

[Mr4D (OOO till 08-26), 2015/03/17 17:04:37]

Practically you do not need a scoped_ptr here - couldn't you add the variable
directly? (BatteryNotification battery_notification_;)

[michaelpg, 2015/03/17 18:32:46]

Hmm. I had trouble including battery_notification.h because it depends on this
file for the NotificationState enum. It doesn't look like we allow C++11 forward
declaration of enums. Am I missing a simple way of resolving the this dependency
issue?

I thought about moving the enum to BatteryNotification. In fact, maybe all the
logic about when to show a notification belongs in BatteryNotification as it
doesn't directly relate to the tray anymore. WDYT? 

[Mr4D (OOO till 08-26), 2015/03/17 22:25:04]

That makes sense! Please do!

[michaelpg, 2015/03/17 23:10:39]

Took another look. TrayPower uses NotificationState to determine when to update
the tray view. I wonder if maybe PowerStatus would be a better place, but I
don't want to add more UI specifics there.

However, maybe a scoped pointer actually works. Why should we keep the
BatteryNotification around when we're not using it? And why instantiate one on
startup unless the battery is low?

BatteryNotification is basically a wrapper around a help function for creating a
notification. Instead of using Show/Hide, we can just create/remove the
notification in the constructor/destructor. Take a look at let me know again
what you think - thanks!

[Mr4D (OOO till 08-26), 2015/03/20 03:10:53]

Okay. Now with your changes it makes sense. Thanks!

   NotificationState notification_state_;
 
   // Was a USB charger connected the last time OnPowerStatusChanged() was