Don't recreate the speech encoder if we don't have to

If the specification for the speech encoder hasn't changed, we should
reuse it instead of recreating it. Otherwise, we lose its state. (This
problem was originally discovered because AudioEncoderOpus instances
would forget that they were supposed to be using DTX.)

BUG=webrtc:6020, chromium:622647

Review-Url: https://codereview.webrtc.org/2089183002
Cr-Commit-Position: refs/heads/master@{#13273}

webrtc/modules/audio_coding/codecs/red/audio_encoder_copy_red.cc

@@ -123,4 +123,9 @@ void AudioEncoderCopyRed::SetTargetBitrate(int bits_per_second) {
   speech_encoder_->SetTargetBitrate(bits_per_second);
 }
 
+rtc::ArrayView<std::unique_ptr<AudioEncoder>>
+AudioEncoderCopyRed::ReclaimContainedEncoders() {
+  return rtc::ArrayView<std::unique_ptr<AudioEncoder>>(&speech_encoder_, 1);
+}
+
 }  // namespace webrtc

webrtc/modules/audio_coding/codecs/red/audio_encoder_copy_red.h

@@ -51,6 +51,8 @@ class AudioEncoderCopyRed final : public AudioEncoder {
   void SetMaxPlaybackRate(int frequency_hz) override;
   void SetProjectedPacketLossRate(double fraction) override;
   void SetTargetBitrate(int target_bps) override;
+  rtc::ArrayView<std::unique_ptr<AudioEncoder>> ReclaimContainedEncoders()
+      override;
 
  protected:
   EncodedInfo EncodeImpl(uint32_t rtp_timestamp,