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

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10168}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc