Add clarifying comment for trusty hack.
Add clarifying comment for trusty hack.

R=dnj@chromium.org, estaab@chromium.org, maruel@chromium.org, vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1434083003

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go