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 9f8ad570d8af7cbe7b57ae5e91b094ffe5670876..53ef52af4771a3db67d0e67132d1579260f40a70 100644
--- a/content/content_renderer.gypi
+++ b/content/content_renderer.gypi
@@ -607,6 +607,8 @@
     # 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',
@@ -775,6 +777,8 @@
     ['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',