Put the close button of a notification into focus chain.

ui/arc/notification/arc_custom_notification_view.h

Index: ui/arc/notification/arc_custom_notification_view.h
diff --git a/ui/arc/notification/arc_custom_notification_view.h b/ui/arc/notification/arc_custom_notification_view.h
index e9820bedabd33c3dc2905a8615d338eaeae46c5d..7eae0013b453de5336d4f8f8a23691b3f9586bd1 100644
--- a/ui/arc/notification/arc_custom_notification_view.h
+++ b/ui/arc/notification/arc_custom_notification_view.h
@@ -21,6 +21,7 @@ class NotificationSurface;
 }
 
 namespace views {
+class FocusTraversable;
 class ImageButton;
 class Widget;
 }
@@ -42,6 +43,7 @@ class ArcCustomNotificationView
 
  private:
   class ContentViewDelegate;
+  class CloseButton;
   class EventForwarder;
   class SlideHelper;
 
@@ -64,6 +66,7 @@ class ArcCustomNotificationView
   void OnMouseExited(const ui::MouseEvent& event) override;
   void OnFocus() override;
   void OnBlur() override;
+  views::FocusTraversable* GetFocusTraversable() override;
 
   // views::ButtonListener
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;