For stabilization purpose, https://chromium-review.googlesource.com/c/499912 introduces an ID for contaner instance.

In the review, Luis proposed to use the ID in the libcontainer, too.
At the moment, we're using mkdtemp to create unique directory for container instance.
So, there's two topics here;

- If we should share the per instance ID between libcontainer (=using it in instance directory path), and communication between Chrome and session_manager.

- If so, by-who/how instance ID should be generated.
  1) If ID is passed to libcontainer from outside, then we need to think about conflicting. (Maybe then it should just return an error?).
  2) If ID is still generated by mkdtemp, then no worry about conflicting because it ensures uniqueness. Then, who owns the responsibility to take the "ID" part from the path, libcontainer or session_manager?

- In case of sharing, it means it exposes container instance path to Chrome. Any security risks (maybe no?)?