bug#37213: Execute set-window-margins. left-margin-width remains 0.

[Eli Zaretskii]

> Date: Wed, 28 Aug 2019 18:53:52 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> emacs -Q
> M-: (set-window-margins (selected-window) 2) ; This works.
> M-: left-margin-width.
> 
> The result is that left-margin-width is still at zero, although the
> value should be 2.  This is a bug.

I don't think this is a bug.  This variable is for _setting_ the
margin width, not for _returning_ its width as set "by other means".
See its documentation.