Fix incorrect use of DCHECK in CommandDictionary::GetCommandsAsJson.

https://weave-review.googlesource.com/c/8458/ introduced a bug in the
CommandDictionary::GetCommandsAsJson, where the |package| variable isn't
updated in the release build due to the incorrect use of DHCECK.

BUG=chromium:691664
TEST=`USE=cros-debug FEATURES=test emerge-arkham libweave`
TEST=`USE=-cros-debug FEATURES=test emerge-arkham libweave`

Change-Id: Ia129019205714c74186b96982fd79cc8a25f9874
Reviewed-on: https://weave-review.googlesource.com/8651
Reviewed-by: Mike Frysinger <vapier@google.com>