switcherPopup: Use roundtrip time when the popup is modifier-less

The noModsTimeout obviously finishes inside a timeout callback, which
means `global.get_current_time()` might return Clutter.CURRENT_TIME (ie.
0) when called inside it, because it's not called while handling an
event. This means when switching apps or activating a window, the
timestamp passed to `activate_window` may be 0, which is the reason why
the altTab switcher is currently broken when using modifier-less
keybindings.

Fix that by using `meta_display_get_current_time_roundtrip`, which
always return a valid timestamp, instead of
`shell_global_get_current_time`.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/847

js/ui/switcherPopup.js

@@ -270,7 +270,7 @@ var SwitcherPopup = GObject.registerClass({
             GLib.PRIORITY_DEFAULT,
             NO_MODS_TIMEOUT,
             () => {
-                this._finish(global.get_current_time());
+                this._finish(global.display.get_current_time_roundtrip());
                 this._noModsTimeoutId = 0;
                 return GLib.SOURCE_REMOVE;
             });