Add support for creating a persistent TUN device (that survives across Bonnet restarts).

gfe-relnote: n/a (QBONE-only change)
PiperOrigin-RevId: 278518919
Change-Id: I38defaf866017445425bf8425b6f3ab98df9a0a4

quic/qbone/bonnet/tun_device.cc

diff --git a/quic/qbone/bonnet/tun_device.cc b/quic/qbone/bonnet/tun_device.cc
index 6c0a8a5..e266654 100644
--- a/quic/qbone/bonnet/tun_device.cc
+++ b/quic/qbone/bonnet/tun_device.cc
@@ -30,7 +30,9 @@
       kernel_(*kernel) {}
 
 TunDevice::~TunDevice() {
-  Down();
+  if (!persist_) {
+    Down();
+  }
   CleanUpFileDescriptor();
 }