Code review comment for lp:~kdub/mir/protobuf-exchange-buffer

We discussed this a bit when I wanted to plan out this line of work on the mailing list (https://lists.ubuntu.com/archives/mir-devel/2014-July/000773.html).

Different buffer sending mechanisms are interesting, but in the name of not rewriting the entire system, I'm going with an incremental improvement on what we already have. My goal here is just to send a file descriptor back with the buffer that the client is returning. I don't want to go too far into buffer sending experiments when my goal is only to implement the fencing improvements that are useful to android.