Consolidate and improve audio decoding test for all decoders.

media/filters/audio_file_reader.cc

Index: media/filters/audio_file_reader.cc
diff --git a/media/filters/audio_file_reader.cc b/media/filters/audio_file_reader.cc
index b20fd8d9ce35795b7ed94e5cea8177d80f0f25f0..4fea1d5274ac371113bb847fcfc3bb84379fada0 100644
--- a/media/filters/audio_file_reader.cc
+++ b/media/filters/audio_file_reader.cc
@@ -257,4 +257,11 @@ bool AudioFileReader::ReadPacket(AVPacket* output_packet) {
   return false;
 }
 
+bool AudioFileReader::SeekForTesting(base::TimeDelta seek_time) {
+  return av_seek_frame(glue_->format_context(),
+                       stream_index_,
+                       ConvertToTimeBase(codec_context_->time_base, seek_time),
+                       AVSEEK_FLAG_BACKWARD) >= 0;
+}
+
 }  // namespace media