Code review comment for lp:~mir-team/mir/handle-mis-shaped-cookies

Hmm that would expose the cookie::Blob :(. This is mainly a ... fix until we move to a std::vector. Once we can replace the android input transport (what ill be looking at soon!). Though it would be nice to return a cookie::Blob from a cookie::serialize()... ideally once we have a std::vector everywhere this function will be the expected public interface.