message-tray: allocate the entire size to the icon

Specify x-fill and y-fill true for the bin that contains the status
icon so the status icon will always be sized to our specified icon
size (24x24). This prevents pathological behavior for legacy status
icons embedded in the tray where an initial allocation at 1x1 before
they had content would "stick", and the icon would permanently
end up 1x1.

https://bugzilla.gnome.org/show_bug.cgi?id=634820

js/ui/messageTray.js

@@ -820,7 +820,9 @@ Source.prototype = {
     _init: function(title) {
         this.title = title;
         this._iconBin = new St.Bin({ width: this.ICON_SIZE,
-                                     height: this.ICON_SIZE });
+                                     height: this.ICON_SIZE,
+                                     x_fill: true,
+                                     y_fill: true });
         this.isTransient = false;
         this.isChat = false;
     },