Set the preferred DSCP value for Rtp data channel to be DSCP_AF41.

webrtc/media/base/rtpdataengine.h

Index: webrtc/media/base/rtpdataengine.h
diff --git a/webrtc/media/base/rtpdataengine.h b/webrtc/media/base/rtpdataengine.h
index 37a84da16fa3b975a78eff3e1be36b1a57f2fc3d..adf0bef6e912a39eeea366f1ab84eccb991a1982 100644
--- a/webrtc/media/base/rtpdataengine.h
+++ b/webrtc/media/base/rtpdataengine.h
@@ -27,7 +27,8 @@ class RtpDataEngine : public DataEngineInterface {
  public:
   RtpDataEngine();
 
-  virtual DataMediaChannel* CreateChannel(DataChannelType data_channel_type);
+  virtual DataMediaChannel* CreateChannel(DataChannelType data_channel_type,
+                                          const MediaConfig& config);
 
   virtual const std::vector<DataCodec>& data_codecs() {
     return data_codecs_;
@@ -61,7 +62,7 @@ class RtpClock {
 
 class RtpDataMediaChannel : public DataMediaChannel {
  public:
-  RtpDataMediaChannel();
+  RtpDataMediaChannel(const MediaConfig& config);
   virtual ~RtpDataMediaChannel();
 
   virtual bool SetSendParameters(const DataSendParameters& params);
@@ -88,6 +89,7 @@ class RtpDataMediaChannel : public DataMediaChannel {
     const SendDataParams& params,
     const rtc::CopyOnWriteBuffer& payload,
     SendDataResult* result);
+  virtual rtc::DiffServCodePoint PreferredDscp() const;
 
  private:
   void Construct();