Pin down app-editors/vim as the virtual/editor in chromeos ebuild.
Some packages, such as sudo, have a dependency on 'virtual/editor'. This can
resolve to different editors depending on what editors are pulled in by other
packages. For example, x86 builds were getting 'nano' and arm builds were
getting 'vim'. To make this more deterministic the chromeos ebuild has been
changed to definitively select vim as the editor of choice.
Note that vim was chosen by the build team lead as the primary editor to be supported on chromeos builds.
TEST=build_image now includes app-editors/vim, with USE=minimal, for chromeos package