Currently, we have three CQ bots that run layout tests:

* linux_chromium_rel_ng (Linux Trusty)
* mac_chromium_rel_ng (Mac 10.12 I think? will be 10.13 soon)
* win7_chromium_rel_ng (layout tests were actually temporarily taken off the bot and are currently experimental because of an infra issue)

So we don't have coverage for other Mac & Windows versions. This is fine generally, but for some CLs (e.g. those changing blinkpy/), we might want to include all Blink try bots to ensure we have full coverage and avoid missing a whole family of OS if any of the three bots is temporarily disabled (like Windows recently).