LogDog: Only create missing hierarchy components.
LogDog: Only create missing hierarchy components.

R=estaab@chromium.org, iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1883593005

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