overview: don't clone the whole message tray when layouting

The message tray actor also includes notifications themselves. We want
our ghost to be sized as the base part of the tray instead.
Just make sure to use the same style class as the base actor then, as
its height is specified by the CSS.

https://bugzilla.gnome.org/show_bug.cgi?id=690174

js/ui/overview.js

@@ -243,7 +243,7 @@ const Overview = new Lang.Class({
                                           expand: true });
 
         // Then account for message tray
-        this._messageTrayGhost = new St.Bin({ child: new Clutter.Clone({ source: Main.messageTray.actor }),
+        this._messageTrayGhost = new St.Bin({ style_class: 'message-tray-summary',
                                               reactive: false,
                                               opacity: 0,
                                               x_fill: true,