libchromeos: Migrate string_utils from Buffet to libchromeos BUG=chromium:405714 TEST=USE=buffet ./build_packages Change-Id: I95d8977aed6ffd718751de058d4cdda3a5395c25 Reviewed-on: https://chromium-review.googlesource.com/213361 Tested-by: Alex Vakulenko <avakulenko@chromium.org> Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org> Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>