Chromium doesn't ship binaries to end users. Linux distros that have chromium build it from source. But I suspect that they are probably going to switch to clang as the recommend compiler for development.
Well, in any case they have a build system in place, I hope :)
AFIAK, Chromium core devs are all Google employees, and their code makes up the base for Chrome, although Chrome is Google branded and includes other features such as usage tracking. I found a talk given in 2011 by two Chromium devs about using clang, which seems to point to nearly the same increase in build performance along with a few other reasons they like using clang. Anyways, seems like they've been using clang for a while. http://llvm.org/devmtg/2011-11/Weber_Wennborg_UsingClangInCh...