Roll chromium revision: 6455c69:2687a76

Pick up the libvpx roll: https://codereview.chromium.org/597703004/

R=johannkoenig@google.com
TBR=ajm@google.com

Review URL: https://webrtc-codereview.appspot.com/23789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7302 4adac7df-926f-26a2-2b94-8c16560cd09d