A seriously minimalistic Pomodoro timer – just a 25 minutes timer, really. Available at https://chrome.google.com/webstore/detail/pomotimer/hefdcogdhfodpbcidnkecllmknddjghc.
Are you on Mac? If so, it's not possible for the extension to enforce a sticky notification. You need to change the notification settings for Chrome to be alerts as opposed to banners in the settings.