Use the same draft version in SDP data channel answers as used in the offer.

webrtc/pc/mediasession.h

Index: webrtc/pc/mediasession.h
diff --git a/webrtc/pc/mediasession.h b/webrtc/pc/mediasession.h
index 06815ecba319970c117318a64d4c6d8f4ea6c3a4..2ce00294348968fe45679e25e8e3d637b64ac957 100644
--- a/webrtc/pc/mediasession.h
+++ b/webrtc/pc/mediasession.h
@@ -401,10 +401,19 @@ class VideoContentDescription : public MediaContentDescriptionImpl<VideoCodec> {
 
 class DataContentDescription : public MediaContentDescriptionImpl<DataCodec> {
  public:
+  DataContentDescription() : use_sctpmap_(true) {}

[Taylor Brandstetter, 2017/02/17 02:10:56]

nit: We generally prefer providing default values where the variable is declared
when it makes sense. So, "bool use_sctpmap_ = true;"

[Zach Stein, 2017/02/17 21:29:17]

Done.

+
   virtual ContentDescription* Copy() const {
     return new DataContentDescription(*this);
   }
   virtual MediaType type() const { return MEDIA_TYPE_DATA; }
+
+  // Maybe it belongs in MediaContentDescription instead

[Taylor Brandstetter, 2017/02/17 02:10:56]

I think it makes sense here, since it's the only place it's relevant.

[Zach Stein, 2017/02/17 21:29:17]

I was thinking about moving it to MediaContentDescription because that would
remove the need to downcast media_desc to data_desc in BuildMediaDescription.
Since I now know that SCTP is only relevant to data channels, I think moving the
call to BuildSctpContentAttributes (and possibly BuildRtpContentAttributes) to
the if (media_type ...) branches of BuildMediaDescription would be better.

+  bool use_sctpmap() const { return use_sctpmap_; }
+  void set_use_sctpmap(bool enable) { use_sctpmap_ = enable; }
+
+ private:
+  bool use_sctpmap_;
 };
 
 // Creates media session descriptions according to the supplied codecs and
@@ -456,9 +465,9 @@ class MediaSessionDescriptionFactory {
       const MediaSessionOptions& options,
       const SessionDescription* current_description) const;
   SessionDescription* CreateAnswer(
-        const SessionDescription* offer,
-        const MediaSessionOptions& options,
-        const SessionDescription* current_description) const;
+      const SessionDescription* offer,
+      const MediaSessionOptions& options,
+      const SessionDescription* current_description) const;
 
  private:
   const AudioCodecs& GetAudioCodecsForOffer(