Added a kEchoCancellation constraint to turn off the audio processing.

content/renderer/media/media_stream_source.h

Index: content/renderer/media/media_stream_source.h
diff --git a/content/renderer/media/media_stream_source.h b/content/renderer/media/media_stream_source.h
index 138e58ec6bd9bfb55442d70ed20b7345c8906b38..18821c38cf5a4fc0c285f49932506ee570273069 100644
--- a/content/renderer/media/media_stream_source.h
+++ b/content/renderer/media/media_stream_source.h
@@ -27,6 +27,10 @@ class CONTENT_EXPORT MediaStreamSource
   typedef base::Callback<void(MediaStreamSource* source,
                               bool success)> ConstraintsCallback;
 
+  // Source constraints key for
+  // http://dev.w3.org/2011/webrtc/editor/getusermedia.html.
+  static const char kSourceId[];
+
   MediaStreamSource();
   virtual ~MediaStreamSource();