ash: Use system tray mojo interface to show system update tray icon

ash/common/system/tray/system_tray_item.h

Index: ash/common/system/tray/system_tray_item.h
diff --git a/ash/common/system/tray/system_tray_item.h b/ash/common/system/tray/system_tray_item.h
index 854bd2342aca980214e68f97285ecf80d60968e0..2e7ca00dd71c2103ba35b3908faf15b58a9bedd8 100644
--- a/ash/common/system/tray/system_tray_item.h
+++ b/ash/common/system/tray/system_tray_item.h
@@ -25,6 +25,10 @@ class SystemTray;
 class SystemTrayBubble;
 class TrayItemView;
 
+// Controller for an item in the system tray. Each item can create these views:
+// Tray view - The icon in the status area in the shelf.
+// Default view - The row in the top-level menu.
+// Detailed view - The submenu shown when the top-level menu row is clicked.
 class ASH_EXPORT SystemTrayItem {
  public:
   // The different types of SystemTrayItems.
@@ -87,6 +91,7 @@ class ASH_EXPORT SystemTrayItem {
 
   // Returns a notification view for the item. This view is displayed with
   // other notifications and should be the same size as default views.
+  // DEPRECATED. Use the message center instead.
   virtual views::View* CreateNotificationView(LoginStatus status);
 
   // These functions are called when the corresponding view item is about to be