In rampup tests, set start time when starting poller thread.

This should avoid perf change caused by
https://codereview.webrtc.org/1378303004

BUG=539257
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1383223002 .

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Cr-Mirrored-Commit: f3a7c9d732bcb4ed59323d78f5e521615a46fdc2
1 file changed