Cease all future TURN requests when a TURN refresh request fails for a given TURN port.
This fixes an assert error in Turnport::OnSendStunPacket

BUG=webrtc:5388

Review URL: https://codereview.webrtc.org/1547373002

Cr-Commit-Position: refs/heads/master@{#11152}
diff --git a/webrtc/p2p/base/stunrequest.h b/webrtc/p2p/base/stunrequest.h
index 15ea0c7..44c1ebf 100644
--- a/webrtc/p2p/base/stunrequest.h
+++ b/webrtc/p2p/base/stunrequest.h
@@ -21,6 +21,8 @@
 
 class StunRequest;
 
+const int kAllRequests = 0;
+
 // Manages a set of STUN requests, sending and resending until we receive a
 // response or determine that the request has timed out.
 class StunRequestManager {
@@ -32,8 +34,10 @@
   void Send(StunRequest* request);
   void SendDelayed(StunRequest* request, int delay);
 
-  // Sends all pending requests right away. Only for testing.
-  void Flush();
+  // If |msg_type| is kAllRequests, sends all pending requests right away.
+  // Otherwise, sends those that have a matching type right away.
+  // Only for testing.
+  void Flush(int msg_type);
 
   // Removes a stun request that was added previously.  This will happen
   // automatically when a request succeeds, fails, or times out.