Printing endpoint

Hi All,

Mozilla has proposed a new endpoint for printing. This will be really
useful for a number of use cases for automation and will be useful for the
printing work happening in the CSS WG.

Please can you either comment on the PR[1] or here. If I don't hear
anything by 18 December I will merge the PR.

Thanks,

David

[1] https://github.com/w3c/webdriver/pull/1468