Revert of Do not activate TrayBubbleView by default

ash/system/tray/system_tray_bubble.h

Index: ash/system/tray/system_tray_bubble.h
diff --git a/ash/system/tray/system_tray_bubble.h b/ash/system/tray/system_tray_bubble.h
index 3ee439d37a9d736ae4b7d1db5be0c986242cb025..4e4c519ff22e55e8bbf2520705517a12f46fb184 100644
--- a/ash/system/tray/system_tray_bubble.h
+++ b/ash/system/tray/system_tray_bubble.h
@@ -37,6 +37,9 @@
   void InitView(views::View* anchor,
                 LoginStatus login_status,
                 views::TrayBubbleView::InitParams* init_params);
+
+  // Focus the default item if no item is focused. Othewise, do nothing.
+  void FocusDefaultIfNeeded();
 
   BubbleType bubble_type() const { return bubble_type_; }
   views::TrayBubbleView* bubble_view() const { return bubble_view_; }