Roll chromium_revision b1748b:681cc8

Pick the libvpx roll: https://codereview.chromium.org/513593002

BUG=3747
R=andrew@webrtc.org
TBR=ajm@google.com

Review URL: https://webrtc-codereview.appspot.com/14229004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6993 4adac7df-926f-26a2-2b94-8c16560cd09d