Add quiet mode settings bubble.

ui/message_center/notification_list.h

Index: ui/message_center/notification_list.h
diff --git a/ui/message_center/notification_list.h b/ui/message_center/notification_list.h
index ce9c26625e00cb3cef4f94f82de5210e6ace5d42..c06fa161c1777ef438929ee95a3665299e93e050 100644
--- a/ui/message_center/notification_list.h
+++ b/ui/message_center/notification_list.h
@@ -133,6 +133,8 @@ class MESSAGE_CENTER_EXPORT NotificationList {
   // Marks the popups returned by GetPopupNotifications() as shown.
   void MarkPopupsAsShown();
 
+  bool quiet_mode() const { return quiet_mode_; }
+
   // Sets the current quiet mode status to |quiet_mode|. The new status is not
   // expired.
   void SetQuietMode(bool quiet_mode);