Fix RtpReceiver.GetParameters when SSRCs aren't signaled.

webrtc/media/engine/webrtcvideoengine2.h

Index: webrtc/media/engine/webrtcvideoengine2.h
diff --git a/webrtc/media/engine/webrtcvideoengine2.h b/webrtc/media/engine/webrtcvideoengine2.h
index 0a2f4a5d86f71906b03939100812ba53aa7f2689..2c7d36ccb5cc820b5d20fa6317d1dbc950ca0f49 100644
--- a/webrtc/media/engine/webrtcvideoengine2.h
+++ b/webrtc/media/engine/webrtcvideoengine2.h
@@ -83,6 +83,9 @@ class DefaultUnsignalledSsrcHandler : public UnsignalledSsrcHandler {
   rtc::VideoSinkInterface<webrtc::VideoFrame>* GetDefaultSink() const;
   void SetDefaultSink(VideoMediaChannel* channel,
                       rtc::VideoSinkInterface<webrtc::VideoFrame>* sink);
+
+  uint32_t default_recv_ssrc() const { return default_recv_ssrc_; }
+
   virtual ~DefaultUnsignalledSsrcHandler() = default;
 
  private: