Show progress bar notifications for Mac.

ui/message_center/cocoa/notification_controller.h

Index: ui/message_center/cocoa/notification_controller.h
diff --git a/ui/message_center/cocoa/notification_controller.h b/ui/message_center/cocoa/notification_controller.h
index 3f05a92b016cd3f8a4abd79ec2ff1c711fb4f5c2..da2dabcbdd2fc83790b6c9a9500caf29e5b58c8f 100644
--- a/ui/message_center/cocoa/notification_controller.h
+++ b/ui/message_center/cocoa/notification_controller.h
@@ -49,6 +49,9 @@ MESSAGE_CENTER_EXPORT
   // Container for optional list item views.
   base::scoped_nsobject<NSView> listItemView_;
 
+  // Container for optional progress bar view.
+  base::scoped_nsobject<NSProgressIndicator> progressBarView_;
+
   // Container for optional items at the bottom of the notification.
   base::scoped_nsobject<NSView> bottomView_;
 }