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-Original-Commit-Position: refs/heads/master@{#10168}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f3a7c9d732bcb4ed59323d78f5e521615a46fdc2
diff --git a/video/rampup_tests.cc b/video/rampup_tests.cc
index 526f4a4..6459c3b 100644
--- a/video/rampup_tests.cc
+++ b/video/rampup_tests.cc
@@ -55,7 +55,7 @@
       start_bitrate_bps_(start_bitrate_bps),
       start_bitrate_verified_(false),
       expected_bitrate_bps_(0),
-      test_start_ms_(clock_->TimeInMilliseconds()),
+      test_start_ms_(-1),
       ramp_up_finished_ms_(-1),
       extension_type_(extension_type),
       ssrcs_(GenerateSsrcs(num_streams, 100)),
@@ -226,6 +226,8 @@
 }
 
 void RampUpTester::TriggerTestDone() {
+  RTC_DCHECK_GE(test_start_ms_, 0);
+
   VideoSendStream::Stats send_stats = send_stream_->GetStats();
 
   size_t total_packets_sent = 0;
@@ -262,6 +264,7 @@
 }
 
 void RampUpTester::PerformTest() {
+  test_start_ms_ = clock_->TimeInMilliseconds();
   poller_thread_->Start();
   if (Wait() != kEventSignaled) {
     printf("Timed out while waiting for ramp-up to complete.");