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

ash/common/system/tray/tray_image_item.h

Index: ash/common/system/tray/tray_image_item.h
diff --git a/ash/common/system/tray/tray_image_item.h b/ash/common/system/tray/tray_image_item.h
index db4c3f0b654dc074f172cc7b36e0c815c82dc607..ad9aa9e0becd02154b6331d7bdd9687263d3b2ec 100644
--- a/ash/common/system/tray/tray_image_item.h
+++ b/ash/common/system/tray/tray_image_item.h
@@ -17,6 +17,7 @@ class ImageView;
 namespace ash {
 class TrayItemView;
 
+// A system tray item that uses an image as its "tray view" in the status area.
 class ASH_EXPORT TrayImageItem : public SystemTrayItem {
  public:
   TrayImageItem(SystemTray* system_tray, int resource_id, UmaType uma_type);
@@ -57,6 +58,7 @@ class ASH_EXPORT TrayImageItem : public SystemTrayItem {
   // The color of the material design icon in the tray.
   SkColor icon_color_;
 
+  // The image view in the tray.
   TrayItemView* tray_view_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayImageItem);