Adjust behavior of control buttons of notifications.

ui/message_center/views/notification_view.h

Index: ui/message_center/views/notification_view.h
diff --git a/ui/message_center/views/notification_view.h b/ui/message_center/views/notification_view.h
index 647dd5747505bc24d6ce958db8c68084353b6403..127b5a91ea61054b718594a03e595c786bde6f81 100644
--- a/ui/message_center/views/notification_view.h
+++ b/ui/message_center/views/notification_view.h
@@ -46,6 +46,8 @@ class MESSAGE_CENTER_EXPORT NotificationView
   void OnFocus() override;
   void ScrollRectToVisible(const gfx::Rect& rect) override;
   gfx::NativeCursor GetCursor(const ui::MouseEvent& event) override;
+  void OnMouseEntered(const ui::MouseEvent& event) override;
+  void OnMouseExited(const ui::MouseEvent& event) override;
 
   // Overridden from MessageView:
   void UpdateWithNotification(const Notification& notification) override;
@@ -53,6 +55,7 @@ class MESSAGE_CENTER_EXPORT NotificationView
   bool IsCloseButtonFocused() const override;
   void RequestFocusOnCloseButton() override;
   bool IsPinned() const override;
+  void UpdateControlButtonsVisibility() override;
 
  protected:
   views::ImageButton* close_button() { return close_button_.get(); }