As ARS Technica points out, this makes Chrome the first major software project to adopt Clang on Windows.
Way back in 2013, Google decided that it wanted to use Clang everywhere, including Windows. Using the same compiler everywhere makes development much easier—you have the same set of bugs to deal with on every platform—and Clang in particular has diagnostic tools such as ASan and UBSan that Google wanted to be able to use.