Allow configuring settings via policies

[septatrix]

Chrome allows not only installing but also configuring extensions using policies. This is very useful when deploying extensions to several machines and performing initial setup.
This would require to declare possible values in the manifest as a schema and then they can be read using the storage.managed API.
My personal use case is setting up kiosk devices to have an on-screen keyboard with the correct layout already set and "Use touch events instead of mouse events" enabled.