Fix NullVideoEngine's CreateChannel implementation.

BUG=
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44149004

Cr-Commit-Position: refs/heads/master@{#8980}

talk/media/base/mediaengine.h

@@ -336,6 +336,7 @@ class NullVideoEngine {
   int GetCapabilities() { return 0; }
   // If you need this to return an actual channel, use FakeMediaEngine instead.
   VideoMediaChannel* CreateChannel(
+      const VideoOptions& options,
       VoiceMediaChannel* voice_media_channel) {
     return NULL;
   }