Introduce AudioBufferMemoryPool to avoid thrashing on audio buffers.

media/filters/ffmpeg_audio_decoder.h

Index: media/filters/ffmpeg_audio_decoder.h
diff --git a/media/filters/ffmpeg_audio_decoder.h b/media/filters/ffmpeg_audio_decoder.h
index 214a2245e4b8696e2f3e52da7f8b8701584a00c5..8ac9d70abf214b6f6cae9b5480681db0d4f7b986 100644
--- a/media/filters/ffmpeg_audio_decoder.h
+++ b/media/filters/ffmpeg_audio_decoder.h
@@ -11,6 +11,7 @@
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/time/time.h"
+#include "media/base/audio_buffer.h"
 #include "media/base/audio_decoder.h"
 #include "media/base/demuxer_stream.h"
 #include "media/base/media_log.h"
@@ -46,6 +47,11 @@ class MEDIA_EXPORT FFmpegAudioDecoder : public AudioDecoder {
               const DecodeCB& decode_cb) override;
   void Reset(const base::Closure& closure) override;
 
+  // Callback called from within FFmpeg to allocate a buffer based on the
+  // properties of |codec_context| and |frame|. See AVCodecContext.get_buffer2
+  // documentation inside FFmpeg.
+  int GetAudioBuffer(struct AVCodecContext* s, AVFrame* frame, int flags);
+
  private:
   // There are four states the decoder can be in:
   //
@@ -111,6 +117,8 @@ class MEDIA_EXPORT FFmpegAudioDecoder : public AudioDecoder {
 
   scoped_refptr<MediaLog> media_log_;
 
+  scoped_refptr<AudioBufferMemoryPool> pool_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(FFmpegAudioDecoder);
 };