This adds the new headless Chromium shell as a browser option. There was some discussion on the headless-chromium merge request about whether we want to do this, particularly since it slows down the seleniumserver compilation by a lot.
This MR includes the patches mentioned in that discussion. I'm not sure if it should be merged, but it ensures that the changes to seleniumserver are visible. There is an example test suite running the ERP5 functional tests on a headless-chromium instance via seleniumserver.