Add protobuf for mime type mappings in Crostini.

BUG=chromium:869747
TEST=Builds

Change-Id: Id2d59d432b8037a4fcc0f2d2c0c267fac5b088f2
Reviewed-on: https://chromium-review.googlesource.com/1188447
Commit-Ready: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>