Use FFmpeg default padding when allocating audio buffers.

media/filters/ffmpeg_audio_decoder.cc

Index: media/filters/ffmpeg_audio_decoder.cc
diff --git a/media/filters/ffmpeg_audio_decoder.cc b/media/filters/ffmpeg_audio_decoder.cc
index 9f5325a24803ed92dc72f67d9a81329c0a6ffc29..14308435c5eeb8e641bc8f28fc80169412ab9877 100644
--- a/media/filters/ffmpeg_audio_decoder.cc
+++ b/media/filters/ffmpeg_audio_decoder.cc
@@ -87,12 +87,9 @@ static int GetAudioBuffer(struct AVCodecContext* s, AVFrame* frame, int flags) {
   // Determine how big the buffer should be and allocate it. FFmpeg may adjust
   // how big each channel data is in order to meet the alignment policy, so
   // we need to take this into consideration.
-  int buffer_size_in_bytes =
-      av_samples_get_buffer_size(&frame->linesize[0],
-                                 channels,
-                                 frame->nb_samples,
-                                 format,
-                                 AudioBuffer::kChannelAlignment);
+  int buffer_size_in_bytes = av_samples_get_buffer_size(
+      &frame->linesize[0], channels, frame->nb_samples, format,
+      0 /* align, use ffmpeg default */);
   // Check for errors from av_samples_get_buffer_size().
   if (buffer_size_in_bytes < 0)
     return buffer_size_in_bytes;