Create field trial for setting a minimum value for Opus encoder packet loss rate

Bug: webrtc:9848
Change-Id: I0663ee3af7729a220de7aff08cd74545e1a7409a
Reviewed-on: https://webrtc-review.googlesource.com/c/104800
Reviewed-by: Minyue Li <minyue@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25081}

modules/audio_coding/codecs/opus/audio_encoder_opus.h

@@ -158,6 +158,7 @@ class AudioEncoderOpusImpl final : public AudioEncoder {
   const bool adjust_bandwidth_;
   bool bitrate_changed_;
   float packet_loss_rate_;
+  const float min_packet_loss_rate_;
   std::vector<int16_t> input_buffer_;
   OpusEncInst* inst_;
   uint32_t first_timestamp_in_buffer_;