Roll libvpx: follow the Chromium revision.

R=andrew@webrtc.org
TBR=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6558 4adac7df-926f-26a2-2b94-8c16560cd09d