Avoid race in RampUpTest
Avoid race in RampUpTest

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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10149}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 20bd750f99bcb510f080999fb8d2296af25d7973

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