Keep on sending stun binding requests on zero-cost networks.
This is useful to keep the NAT binding alive on backup connections.

BUG=

Review URL: https://codereview.webrtc.org/1737493004

Cr-Commit-Position: refs/heads/master@{#11862}
diff --git a/webrtc/p2p/base/stunrequest.cc b/webrtc/p2p/base/stunrequest.cc
index ce0364e..3338861 100644
--- a/webrtc/p2p/base/stunrequest.cc
+++ b/webrtc/p2p/base/stunrequest.cc
@@ -63,6 +63,16 @@
   }
 }
 
+bool StunRequestManager::HasRequest(int msg_type) {
+  for (const auto kv : requests_) {
+    StunRequest* request = kv.second;
+    if (msg_type == kAllRequests || msg_type == request->type()) {
+      return true;
+    }
+  }
+  return false;
+}
+
 void StunRequestManager::Remove(StunRequest* request) {
   ASSERT(request->manager() == this);
   RequestMap::iterator iter = requests_.find(request->id());