Fix flaky ramp-up test.

Don't require the first estimate to be less than the target bitrate. There are other tests verifying that BWE works, so it's enough for this test to measure the
time it takes to ramp-up.

R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20979004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6764 4adac7df-926f-26a2-2b94-8c16560cd09d