message: Launch external ref control for vp9 encoder
  Change field trial condition to killswitch instead.
  Finch trial is going to 100% public today.
  Cr-Commit-Position: refs/heads/master@{#31122}
diff: @@ -582,7 +582,8 @@ int VP9EncoderImpl::InitEncode(const VideoCodec* inst,
  // External reference control is required for different frame rate on spatial
  // layers because libvpx generates rtp incompatible references in this case.
  - external_ref_control_ = field_trial::IsEnabled("WebRTC-Vp9ExternalRefCtrl") ||
  + external_ref_control_ =
  + !field_trial::IsDisabled("WebRTC-Vp9ExternalRefCtrl") ||
  (num_spatial_layers_ > 1 &&
  codec_.mode == VideoCodecMode::kScreensharing) ||
  inter_layer_pred_ == InterLayerPredMode::kOn;
  