chromium/chrome/browser/apps/browser_instance/README

This folder contains the functionality for tracking browser app instances, that
is, tabs opened in Chrome (and are part of the tabbed browser) that are
logically considered part of an app. This functionality is only used on
ChromeOS, which has "tabbed apps" as part of its UX where URLs in the browser
can have first-class OS UX, e.g. icons in the shelf and launcher. This code
is required to ensure those UX work correctly.