ash: Update system update notification behaviour.

ash/system/tray/tray_image_item.cc

Index: ash/system/tray/tray_image_item.cc
diff --git a/ash/system/tray/tray_image_item.cc b/ash/system/tray/tray_image_item.cc
index fbef4205e933dc9c3e375f2a0a906186ef3ad51e..07a48ececcbcfc9bd76fb6d5b9e562e674946667 100644
--- a/ash/system/tray/tray_image_item.cc
+++ b/ash/system/tray/tray_image_item.cc
@@ -25,6 +25,14 @@ views::View* TrayImageItem::tray_view() {
   return tray_view_;
 }
 
+void TrayImageItem::UpdateResourceId(int resource_id) {
+  resource_id_ = resource_id;
+  if (!tray_view_)
+    return;
+  tray_view_->image_view()->SetImage(ui::ResourceBundle::GetSharedInstance().
+      GetImageNamed(resource_id_).ToImageSkia());
+}
+
 views::View* TrayImageItem::CreateTrayView(user::LoginStatus status) {
   CHECK(tray_view_ == NULL);
   tray_view_ = new TrayItemView;