Poller thread is currently started in the constructor, so the first call
to PollStats() may happen even before the streams have been configured.
This will blow up on RTC_DCHECK_GT(expected_bitrate_bps_, 0);
Thread should instead be started on PerformTest() call.
BUG=
Review URL: https://codereview.webrtc.org/1378303004
Cr-Commit-Position: refs/heads/master@{#10149}