NetEq fix for repeated audio issue.

This CL implements a fix behind a field trial for a NetEq issue. NetEq restarts audio too quickly after a buffer underrun, which can quickly lead to another underrun in some circumstances. The fix changes NetEq's behavior to wait with restarting playback until sufficient audio is buffered.

Bug: webrtc:9289
Change-Id: I5968c9478ce8d84caf77f00b8d0a39156b47fc8d
Reviewed-on: https://webrtc-review.googlesource.com/77423
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23347}

api/audio_codecs/audio_decoder.cc

@@ -51,6 +51,10 @@ class OldStyleEncodedFrame final : public AudioDecoder::EncodedAudioFrame {
 
 }  // namespace
 
+bool AudioDecoder::EncodedAudioFrame::IsDtxPacket() const {
+  return false;
+}
+
 AudioDecoder::ParseResult::ParseResult() = default;
 AudioDecoder::ParseResult::ParseResult(ParseResult&& b) = default;
 AudioDecoder::ParseResult::ParseResult(uint32_t timestamp,

api/audio_codecs/audio_decoder.h

@@ -48,6 +48,9 @@ class AudioDecoder {
     // If no duration can be ascertained, returns zero.
     virtual size_t Duration() const = 0;
 
+    // Returns true if this packet contains DTX.
+    virtual bool IsDtxPacket() const;
+
     // Decodes this frame of audio and writes the result in |decoded|.
     // |decoded| must be large enough to store as many samples as indicated by a
     // call to Duration() . On success, returns an rtc::Optional containing the