libchromeos: Move mime_utils from Buffet to libchromeos BUG=chromium:405714 TEST=USE=buffet ./build_packages Change-Id: I4e0bf9600aedda9a9ac913a1234ab1b20da72777 Reviewed-on: https://chromium-review.googlesource.com/213364 Tested-by: Alex Vakulenko <avakulenko@chromium.org> Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org> Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>