This directory should contain Ash 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.
Most of this code originally came from chrome/browser/chromeos as part of a refactoring that split Ash code from the Lacros browser code. See the Lacros project and the “Lacros: ChromeOS source code directory migration” design doc at https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.