message: Add BW limited vp9 k-svc test
  This test would've cought the regression leading to chrome crashes.
  Cr-Commit-Position: refs/heads/master@{#30606}
diff: @@ -1170,6 +1170,23 @@ TEST(FullStackTest, VP9KSVC_3SL_Low) {
  fixture->RunWithAnalyzer(simulcast);
  }
  +TEST(FullStackTest, VP9KSVC_3SL_Low_Bw_Limited) {
  + webrtc::test::ScopedFieldTrials override_trials(
  + AppendFieldTrials("WebRTC-Vp9IssueKeyFrameOnLayerDeactivation/Enabled/"
  + "WebRTC-Vp9ExternalRefCtrl/Enabled/"));
  + auto fixture = CreateVideoQualityTestFixture();
  + ParamsWithLogging simulcast;
  + simulcast.config->link_capacity_kbps = 500;
  + simulcast.call.send_side_bwe = true;
  + simulcast.video[0] = SvcVp9Video();
  + simulcast.analyzer = {"vp9ksvc_3sl_low_bw_limited", 0.0, 0.0,
  + kFullStackTestDurationSecs};
  + simulcast.ss[0] = {
  + std::vector<VideoStream>(), 0, 3, 0, InterLayerPredMode::kOnKeyPic,
  + std::vector<SpatialLayer>(), false};
  + fixture->RunWithAnalyzer(simulcast);
  +}
  +
  TEST(FullStackTest, VP9KSVC_3SL_Medium_Network_Restricted) {
  webrtc::test::ScopedFieldTrials override_trials(
  AppendFieldTrials("WebRTC-Vp9IssueKeyFrameOnLayerDeactivation/Enabled/"));
  