common.proto: Add DLC image type

BUG=chromium:1122854
TEST=./generate.py

Change-Id: I19c6b2ad6d3597bd83155802b21e46a06c247e16
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2536805
Commit-Queue: George Engelbrecht <engeg@google.com>
Commit-Queue: Navil Perez <navil@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
Reviewed-by: Navil Perez <navil@google.com>
2 files changed