Free GPS Tracker by GEOLINK allows to track mobile phones in the browser. Simply open free account on https://geolink.io, install this app and start tracking.
Detailed manual:
1. Install the app, click "Start Tracking"
2. Open free account on https://geolink.io/register.php
3. Your phones ID will be displayed in the app, just activate it on geolink.io by following this guide: https://geolink.io/guide.php
Note: Continued use of GPS running in the background can dramatically decrease battery life.