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

For explanation of what this is, see:
https://codereview.webrtc.org/2063823008/

R=glaznev@webrtc.org, pthatcher@webrtc.org
TBR=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/2107303003 .

Cr-Commit-Position: refs/heads/master@{#13366}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index 3757dad..e28a27e 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -301,6 +301,9 @@
     rtc::Optional<bool> enable_dtls_srtp;
     int ice_candidate_pool_size = 0;
     bool prune_turn_ports = false;
+    // If set to true, this means the ICE transport should presume TURN-to-TURN
+    // candidate pairs will succeed, even before a binding response is received.
+    bool presume_writable_when_fully_relayed = false;
   };
 
   struct RTCOfferAnswerOptions {