This change includes a rewrite of the previous
component/headless-chromium, which used an outdated version of Chromium.
It differs from component/chromium in that:

- It only provides a headless shell, rather than the full Chromium
  browser (though this can be changed).
- It compiles from source rather than downloading a binary build.

There is also a new software release software/headless-chromium which
makes use of the updated component/headless-chromium to expose a remote
debugging port.

See also nexedi/slapos!1014.