Adjust/increase rate control thresold for a vp9 test.

TBR=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/25049004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7589 4adac7df-926f-26a2-2b94-8c16560cd09d