Re-enable the BitrateEstimatorTest cases for the Call API.

BUG=
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18439004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6141 4adac7df-926f-26a2-2b94-8c16560cd09d