Add AudioTrackRecorder for audio component of MediaStream recording.

content/content_renderer.gypi

Index: content/content_renderer.gypi
diff --git a/content/content_renderer.gypi b/content/content_renderer.gypi
index 5e0f0016f5b52fe8538a9da601319a2f0f029735..e33388727c04d87250c1ba5bc20e8e125accd801 100644
--- a/content/content_renderer.gypi
+++ b/content/content_renderer.gypi
@@ -617,6 +617,8 @@
     # WebRTC-specific sources. Put WebRTC plugin-related stuff in the
     # Plugin+WebRTC section below.
     'private_renderer_webrtc_sources': [
+      'renderer/media/audio_track_recorder.cc',
+      'renderer/media/audio_track_recorder.h',
       'renderer/media/media_recorder_handler.cc',
       'renderer/media/media_recorder_handler.h',
       'renderer/media/media_stream.cc',
@@ -789,6 +791,8 @@
     ['OS=="android"', {
       'sources!': [
         'renderer/media/audio_decoder.cc',
+        'renderer/media/audio_track_recorder.cc',
+        'renderer/media/audio_track_recorder.h',
         'renderer/media/media_recorder_handler.cc',
         'renderer/media/media_recorder_handler.h',
         'renderer/media/video_track_recorder.cc',