Add a SetDoNotFragment() method to DatagramSocket, and call this for

net/socket/socket_test_util.cc

Index: net/socket/socket_test_util.cc
diff --git a/net/socket/socket_test_util.cc b/net/socket/socket_test_util.cc
index 58c1d2104adaab66ab3861b29f944565142c096c..2ca14e2ab66560f8bbda0b8376c0fdc76784e3e6 100644
--- a/net/socket/socket_test_util.cc
+++ b/net/socket/socket_test_util.cc
@@ -1331,6 +1331,10 @@ int MockUDPClientSocket::SetSendBufferSize(int32_t size) {
   return OK;
 }
 
+int MockUDPClientSocket::SetDoNotFragment() {
+  return OK;
+}
+
 void MockUDPClientSocket::Close() {
   connected_ = false;
 }