we're currently updating all our Linux bots from Trusty to Xenial (tracked in https://crbug.com/954450
). Once that's done, we're going to build our prebuilt clang (and other prebuilt binaries) on Xenial, so to build you will need to use Xenial or newer. If you have a bunch of Linux bots that build Chromium, you will want to make sure they're on Xenial or newer (or on something with comparable libstdc++ / glibc / etc versions if you're on a different distro) as well.
I expect that this will take us at least another two weeks, and possibly longer.
If you don't use our prebuilt clang / gn / ninja (etc) binaries and instead build them from source, this won't affect you. (One of the motivations here is that clang now needs at least gcc5.1 as a host compiler, so will need a new enough host gcc for building clang in this case.)
This does not affect running chrome. We're not changing runtime requirements of chrome. This is only about system requirements for building chrome.