rpcexplorer: use Roboto font
rpcexplorer: use Roboto font

R=andybons@chromium.org

Review URL: https://codereview.chromium.org/1753673003

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