Remove the most obvious dead code from message center.

ash/system/web_notification/web_notification_tray.h

Index: ash/system/web_notification/web_notification_tray.h
diff --git a/ash/system/web_notification/web_notification_tray.h b/ash/system/web_notification/web_notification_tray.h
index 999657ede20442bca609eee789e06c202a61dd2d..6152122c93dbf23f182bd3850e8d2d34568cefac 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -33,7 +33,6 @@ namespace message_center {
 class MessageBubbleBase;
 class MessageCenter;
 class MessageCenterBubble;
-class MessagePopupBubble;
 class MessagePopupCollection;
 }
 
@@ -145,7 +144,6 @@ class ASH_EXPORT WebNotificationTray
   // Testing accessors.
   bool IsPopupVisible() const;
   message_center::MessageCenterBubble* GetMessageCenterBubbleForTest();
-  message_center::MessagePopupBubble* GetPopupBubbleForTest();
 
   scoped_ptr<message_center::MessageCenterTray> message_center_tray_;
   scoped_ptr<internal::WebNotificationBubbleWrapper> message_center_bubble_;