Fix windows build break

TBR=pthacther@webrtc.org
BUG=4576

Review URL: https://codereview.webrtc.org/1171483004

Cr-Commit-Position: refs/heads/master@{#9372}
diff --git a/webrtc/p2p/stunprober/stunprober.h b/webrtc/p2p/stunprober/stunprober.h
index 30cfdb5..e49b5f7 100644
--- a/webrtc/p2p/stunprober/stunprober.h
+++ b/webrtc/p2p/stunprober/stunprober.h
@@ -274,8 +274,8 @@
 
   int GetTotalClientSockets() { return 1; }
   int GetTotalServerSockets() {
-    return (shared_socket_mode_ ? 1 : all_servers_ips_.size()) *
-           requests_per_ip_;
+    return static_cast<int>(
+        (shared_socket_mode_ ? 1 : all_servers_ips_.size()) * requests_per_ip_);
   }
 
   bool SendNextRequest();