This directory contains files for managing ChromeOS users. Historically, the code manages both user and user sessions. There is an on-going effort to move user session related code into //components/session_manager.
UserManager is the interface for managing ChromeOS users. UserManagerBase is a base implementation of the interface. There is also a UserManagerInterface in Chrome code that provides additional UserManager interface that deals with policy. ChromeUserManager inherits UserManagerBase and UserManagerInterface it provide a base implementation. Finally, the concrete instance used is ChromeUserManagerImpl derived from ChromeUserManager.
ChromeUserManagerImpl is created at the PreProfileInit stage and destroyed at the PostMainMessageLoopRun stage, via BrowserProcessPlatformPart::InitializeChromeUserManager() and BrowserProcessPlatformPart::DestroyChromeUserManager.