wpt/css directory renaming underway; messy

[Philip Jägenstedt]

Hi all,

Please be aware that I'm working on https://bugs.chromium.org/p/chromium/issues/detail?id=780700 so you'll see lots of changes to things that look interesting, and moreover it will be over multiple CLs so things will be quite messy in the interim.

Once done, however, there will be no more versioned directories, everything will be in external/wpt/css/ with directories named by spec. I hope this'll be a productivity boost for anyone who's working with wpt, which I hope is everyone, all the time :)