webrtc/overrides: add OWNERS-file.

BUG=N/A
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7205 4adac7df-926f-26a2-2b94-8c16560cd09d