MessageCenter::RemoveNotification and MessageCenterObserver::OnNotificationRemoved have a boolean argument |by_user|.

|by_user| is not granular enough, and the meaning is ambiguous. Also it's not documented.

Suggested in the CL https://crrev.com/c/882622 :

> It might also make sense for RemoveNotification/OnNotificationRemoved to have an Event* param instead of |by_user|, which is apparently not granular enough.
> you can give the event a default value of nullptr so that the majority of callsites don't have this distracting "false /* by_user */".

Yoshiki also mentioned offline that it was on his potential improvement list.