Introduce struct MediaConfig, with construction-time settings.

webrtc/media/webrtc/nullwebrtcvideoengine.h

Index: webrtc/media/webrtc/nullwebrtcvideoengine.h
diff --git a/webrtc/media/webrtc/nullwebrtcvideoengine.h b/webrtc/media/webrtc/nullwebrtcvideoengine.h
index 4f9d2d9c6de19911c2b80039f04c46c56b7ec902..4b1d3b3d03dd4804e7abca1a195844a40de50524 100644
--- a/webrtc/media/webrtc/nullwebrtcvideoengine.h
+++ b/webrtc/media/webrtc/nullwebrtcvideoengine.h
@@ -68,6 +68,7 @@ class NullWebRtcVideoEngine {
   }
 
   VideoMediaChannel* CreateChannel(webrtc::Call* call,
+      const MediaConfig& config,
       const VideoOptions& options) {
     return nullptr;
   }