Code review comment for lp:~vanvugt/mir/remove-unused-session-param

I want it for in-progress branch https://code.launchpad.net/~mterry/unity-system-compositor/greeter-depth/+merge/207549

The idea is that I can inspect surface name at surface creation time, to know whether the surface belongs to a greeter or user session, to be able to properly assign depth ID.

If there's now a better way to do such filtering, let me know. But the parameter *is* being used by me right now.