Ever since updating to Chromium v65 or 18.04 (not sure which one is the culprit), unchecking the "Use system title bar and borders" doesn't work anymore. The title bar will shows up once I unmaximize it, GTK or classic theme.
If you use Compiz, Chromium's title bar can be hidden by not decorating it.
To do this, go to CompizConfig Settings Manager, scroll down to the "Effects" section and click "Window Decorations". On the "Decoration windows" field, replace
modified from this guide.