package_to_container: generate chrome extension manifest

package_to_container will now produce a manifest that can be
used to identify the container when packaged into a CRX file.

BUG=chromium:644347
TEST=check for manifest.json in the output directory, try to
  mount using mount_extension_image on a DUT and ensure it
  finds the container extension

Change-Id: I7f5b01fa13de54218b9d4dc20774168bc875ee70
Reviewed-on: https://chromium-review.googlesource.com/471786
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
1 file changed