Add signaling to support ICE renomination.
By default, this will tell the remote side that I am supporting ICE renomination.
It does not use ICE renomination yet even if the remote side supports it.

R=deadbeef@webrtc.org, pthatcher@webrtc.org, skvlad@webrtc.org

Review URL: https://codereview.webrtc.org/2224563004 .

Cr-Commit-Position: refs/heads/master@{#13998}
diff --git a/webrtc/api/peerconnection.h b/webrtc/api/peerconnection.h
index c4a9a60..042e7f1 100644
--- a/webrtc/api/peerconnection.h
+++ b/webrtc/api/peerconnection.h
@@ -239,6 +239,9 @@
       const RTCOfferAnswerOptions& options,
       cricket::MediaSessionOptions* session_options);
 
+  void InitializeOptionsForAnswer(
+      cricket::MediaSessionOptions* session_options);
+
   // Helper function for options processing.
   // Deprecated.
   virtual void FinishOptionsForAnswer(
@@ -414,6 +417,8 @@
 
   bool remote_peer_supports_msid_ = false;
 
+  bool enable_ice_renomination_ = false;
+
   std::vector<rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>>>
       senders_;
   std::vector<