Don't use libvpx if it is disabled in GN build

remoting/protocol/session_config.cc

Index: remoting/protocol/session_config.cc
diff --git a/remoting/protocol/session_config.cc b/remoting/protocol/session_config.cc
index 1728a808129092fd66c7fb4ae66786dd29b73a46..3f2c911b0c1db8da62e43a9f0e4a6c4d1d2aed39 100644
--- a/remoting/protocol/session_config.cc
+++ b/remoting/protocol/session_config.cc
@@ -192,10 +192,12 @@ scoped_ptr<CandidateSessionConfig> CandidateSessionConfig::CreateDefault() {
                     ChannelConfig::CODEC_UNDEFINED));
 
   // Video channel.
+#if !defined(MEDIA_DISABLE_LIBVPX)
   result->mutable_video_configs()->push_back(
       ChannelConfig(ChannelConfig::TRANSPORT_STREAM,
                     kDefaultStreamVersion,
                     ChannelConfig::CODEC_VP8));
+#endif  // !defined(MEDIA_DISABLE_LIBVPX)
 
   // Audio channel.
   result->mutable_audio_configs()->push_back(