Introduce AudioClock to improve playback delay calculations.

media/filters/audio_renderer_impl.h

Index: media/filters/audio_renderer_impl.h
diff --git a/media/filters/audio_renderer_impl.h b/media/filters/audio_renderer_impl.h
index f6f41f1ea5b3ec741a7706ce860f98f9bc51a4be..4e3531be795d30ca5a691edd3798bbcd83fa7b61 100644
--- a/media/filters/audio_renderer_impl.h
+++ b/media/filters/audio_renderer_impl.h
@@ -37,11 +37,12 @@ class SingleThreadTaskRunner;
 
 namespace media {
 
-class AudioBus;
 class AudioBufferConverter;
+class AudioBus;
+class AudioHardwareConfig;
 class AudioSplicer;
+class BufferedAudioTracker;
 class DecryptingDemuxerStream;
-class AudioHardwareConfig;
 
 class MEDIA_EXPORT AudioRendererImpl
     : public AudioRenderer,
@@ -276,6 +277,8 @@ class MEDIA_EXPORT AudioRendererImpl
   // false otherwise. This flag is cleared on the next Preroll() call.
   bool preroll_aborted_;
 
+  scoped_ptr<BufferedAudioTracker> buffered_audio_tracker_;
+
   // End variables which must be accessed under |lock_|. ----------------------
 
   // NOTE: Weak pointers must be invalidated before all other member variables.