Fix DCHECK in the chromoting host when disconnecting a WebRTC connection.

remoting/protocol/webrtc_transport.cc

Index: remoting/protocol/webrtc_transport.cc
diff --git a/remoting/protocol/webrtc_transport.cc b/remoting/protocol/webrtc_transport.cc
index fbacd2c418048397a49b83abb67c1c7f25da40c2..7fc239f23561f4466d4f8e11a0ad3a97c1c3b5ad 100644
--- a/remoting/protocol/webrtc_transport.cc
+++ b/remoting/protocol/webrtc_transport.cc
@@ -208,6 +208,9 @@ class WebrtcTransport::PeerConnectionWrapper
     // See crbug.com/660081.
     base::ThreadRestrictions::ScopedAllowIO allow_io;
     peer_connection_->Close();
+    peer_connection_ = nullptr;
+    peer_connection_factory_ = nullptr;
+    audio_module_ = nullptr;
   }
 
   WebrtcAudioModule* audio_module() {