Adding API for "presume writable when fully relayed" ICE option.

webrtc/api/webrtcsession.cc

Index: webrtc/api/webrtcsession.cc
diff --git a/webrtc/api/webrtcsession.cc b/webrtc/api/webrtcsession.cc
index 9801d58f174939ded34c1fdd99261f798cb21e4d..231d0ad4dee1bf730ee79cd856353ad9cef43474 100644
--- a/webrtc/api/webrtcsession.cc
+++ b/webrtc/api/webrtcsession.cc
@@ -1136,6 +1136,8 @@ cricket::IceConfig WebRtcSession::ParseIceConfig(
       config.ice_backup_candidate_pair_ping_interval;
   ice_config.gather_continually = (config.continual_gathering_policy ==
                                    PeerConnectionInterface::GATHER_CONTINUALLY);
+  ice_config.presume_writable_when_fully_relayed =
+      config.presume_writable_when_fully_relayed;
   return ice_config;
 }