Tighten up MSan blacklist.txt owners.

To avoid people adding stuff to the blacklist unless
it's really valid to do so.

BUG=
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29859004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7492 4adac7df-926f-26a2-2b94-8c16560cd09d