Fixes log spam introduced with r6041.

We shouldn't return an error if we don't yet have a valid estimate.

BUG=crbug/371714
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15469006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6111 4adac7df-926f-26a2-2b94-8c16560cd09d