Fix meta_window_titlebar_is_onscreen() for titlebar-less windows

Make the code correspond to the comment - the titlebar can't be
offscreen if there's no titlebar.

https://bugzilla.gnome.org/show_bug.cgi?id=723580

src/core/window.c

@@ -9005,7 +9005,7 @@ meta_window_titlebar_is_onscreen (MetaWindow *window)
 
   /* Titlebar can't be offscreen if there is no titlebar... */
   if (!window->frame)
-    return FALSE;
+    return TRUE;
 
   /* Get the rectangle corresponding to the titlebar */
   meta_window_get_frame_rect (window, &titlebar_rect);