Avoid constraint version of PeerConnection method CreateAnswer

Bug: webrtc:9239
Change-Id: I796ec3f6b332233c20050e58fc1dd97730dcbbe7
Reviewed-on: https://chromium-review.googlesource.com/1113453Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Niels Möller <nisse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570343}

remoting/protocol/webrtc_transport.cc

@@ -559,11 +559,12 @@ void WebrtcTransport::OnRemoteDescriptionSet(bool send_answer,
 
   // Create and send answer on the server.
   if (send_answer) {
+    const webrtc::PeerConnectionInterface::RTCOfferAnswerOptions options;
     peer_connection()->CreateAnswer(
         CreateSessionDescriptionObserver::Create(
             base::Bind(&WebrtcTransport::OnLocalSessionDescriptionCreated,
                        weak_factory_.GetWeakPtr())),
-        nullptr);
+        options);
   }
 
   AddPendingCandidatesIfPossible();