forgot to add shutdown to other platforms (re. https://github.com/gorhill/uMatrix/issues/144)

platform/chromium/vapi-client.js

@@ -167,6 +167,8 @@ vAPI.messaging = {
     }
 };
 
+/******************************************************************************/
+
 // No need to have vAPI client linger around after shutdown if
 // we are not a top window (because element picker can still
 // be injected in top window).