Fix test race in GetStatsMultipleSendStreams.

Test now waits for stats to be filled instead of failing instantly if
they haven't been updated.

BUG=2409
R=asapersson@webrtc.org
TBR=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36239004

Cr-Commit-Position: refs/heads/master@{#8441}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8441 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed