Add additional metric (relative standard deviation of encode time) for overuse detection.
This code is currently only for testing.

BUG=1577
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6381 4adac7df-926f-26a2-2b94-8c16560cd09d