main: move pointer-sync-on-restack code here

We call global.sync_pointer() on MetaScreen::restack as a hack to try
to fix up the hover state after a pointer grab. Previously we were
doing this in chrome.js, since there was already a ::restack handler
there anyway, but this isn't really related to the chrome at all, so
move it to main.js instead.

https://bugzilla.gnome.org/show_bug.cgi?id=633620

js/ui/chrome.js

@@ -335,11 +335,6 @@ Chrome.prototype = {
             this._updateVisibility();
             this._queueUpdateRegions();
         }
-
-        // Figure out where the pointer is in case we lost track of
-        // it during a grab. (In particular, if a trayicon popup menu
-        // is dismissed, see if we need to close the message tray.)
-        global.sync_pointer();
     },
 
     _updateRegions: function() {