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 29feef129e27265134004776d4a735e5eb34c21e..54ed27dbdbb0f440aea7aa0054b282744982d0c4 100644
--- a/webrtc/media/webrtc/nullwebrtcvideoengine.h
+++ b/webrtc/media/webrtc/nullwebrtcvideoengine.h
@@ -51,7 +51,8 @@ class NullWebRtcVideoEngine {
   }
 
   VideoMediaChannel* CreateChannel(webrtc::Call* call,
-      const VideoOptions& options) {
+                                   const MediaConfig& config,
+                                   const VideoOptions& options) {
     return nullptr;
   }