This directory should contain non-UI Chrome OS specific code that has chrome/browser dependencies.
The code in this directory should live in namespace ash. While code in //chrome is not supposed to be in any namespace, //chrome/browser/ash is technically part of the ash binary. The fact that it lives in //chrome/browser instead of in //ash is because top level product directories shouldn't be depended on by any other directory. In the future, when some of the dependencies from //chrome/browser/ash to //chrome/browser are sorted out, some of this code will move to //ash.
As of January 2021, code from chrome/browser/chromeos is migrating into this directory, as part of the Lacros project.
Googlers: See go/lacros-directory-migration for more details.