Fix incorrect opus seek preroll and flaky pre-skip removal.

media/filters/audio_decoder_unittest.cc

Index: media/filters/audio_decoder_unittest.cc
diff --git a/media/filters/audio_decoder_unittest.cc b/media/filters/audio_decoder_unittest.cc
index d428107519542b38db5e31283d6bd255b9012e51..3be1ee8b7e23bd66022679607cde143408fee6a3 100644
--- a/media/filters/audio_decoder_unittest.cc
+++ b/media/filters/audio_decoder_unittest.cc
@@ -64,6 +64,8 @@ std::ostream& operator<<(std::ostream& os, const DecoderTestData& data) {
 // discard metadata in favor of setting DiscardPadding on the DecoderBuffer.
 // Allows better testing of AudioDiscardHelper usage.
 static void SetDiscardPadding(AVPacket* packet,
+                              bool first_packet,
+                              const AVStream* stream,
                               const scoped_refptr<DecoderBuffer> buffer,
                               double samples_per_second) {
   // Discard negative timestamps.
@@ -79,15 +81,20 @@ static void SetDiscardPadding(AVPacket* packet,
   }
 
   // If the timestamp is positive, try to use FFmpeg's discard data.
+  int skip_samples = 0;
   int skip_samples_size = 0;
   const uint32* skip_samples_ptr =
       reinterpret_cast<const uint32*>(av_packet_get_side_data(
           packet, AV_PKT_DATA_SKIP_SAMPLES, &skip_samples_size));
-  if (skip_samples_size < 4)
+  if (skip_samples_size >= 4)
+    skip_samples = base::ByteSwapToLE32(*skip_samples_ptr);
+  else if (first_packet && stream->codec->delay)
+    skip_samples = stream->codec->delay;
+  else
     return;
+
   buffer->set_discard_padding(std::make_pair(
-      base::TimeDelta::FromSecondsD(base::ByteSwapToLE32(*skip_samples_ptr) /
-                                    samples_per_second),
+      base::TimeDelta::FromSecondsD(skip_samples / samples_per_second),
       base::TimeDelta()));
 }
 
@@ -188,10 +195,9 @@ class AudioDecoderTest : public testing::TestWithParam<DecoderTestData> {
     if (packet.flags & AV_PKT_FLAG_KEY)
       buffer->set_is_key_frame(true);
 
-    // Don't set discard padding for Opus, it already has discard behavior set
-    // based on the codec delay in the AudioDecoderConfig.
-    if (GetParam().decoder_type == FFMPEG)
-      SetDiscardPadding(&packet, buffer, GetParam().samples_per_second);
+    SetDiscardPadding(&packet, buffer->timestamp() == start_timestamp_,
+                      reader_->GetAVStreamForTesting(), buffer,
+                      GetParam().samples_per_second);
 
     // DecodeBuffer() shouldn't need the original packet since it uses the copy.
     av_free_packet(&packet);