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>
4 files changed