libtailscale: do not close old TUNs during reconfig on ChromeOS

The bug that required us to close previous tunnels during VPN reconfiguration
on ChromeOS in 8dbac87 was resolved
sometime between June 2020 and February 2021.

While we do not know exactly when it was fixed, we believe the issue was resolved
in November 2020, when OpenVpnDriver was migrated to the VpnService state machine:
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2497361

While I couldn't find a reliable way to determine which ChromeOS version we're running on,
the Auto Update Expiration (AUE) policy suggests that only devices manufactured in 2017
or earlier might not be eligible for an update to a version with the seamless handover fix.

Considering the above, in this commit we stop closing old TUNs on ChromeOS to avoid
breaking connections for users running newer (November 2020+) ChromeOS releases.

Updates tailscale/tailscale#19591

Signed-off-by: Nick Khyl <nickk@tailscale.com>

Author: nickkhyl

libtailscale/net.go

@@ -80,16 +80,7 @@ func (b *backend) updateTUN(rcfg *router.Config, dcfg *dns.OSConfig) (err error)
 	b.logger.Logf("updateTUN: changed")
 	defer b.logger.Logf("updateTUN: finished")
 
-	hasTunnel := false
-	disableTUN := len(rcfg.LocalAddrs) == 0
-	isChromeOS, _ := b.appCtx.IsChromeOS()
-	if disableTUN || isChromeOS {
-		// Close previous tunnel(s).
-		// This is necessary for ChromeOS, native Android devices
-		// seem to handle seamless handover between tunnels correctly.
-		//
-		// TODO(eliasnaur): If seamless handover becomes a desirable feature, skip
-		// the closing on ChromeOS.
+	if disableTUN := len(rcfg.LocalAddrs) == 0; disableTUN {
 		b.logger.Logf("updateTUN: closing old TUNs")
 		b.CloseTUNs()
 		b.logger.Logf("updateTUN: closed old TUNs")
@@ -104,14 +95,9 @@ func (b *backend) updateTUN(rcfg *router.Config, dcfg *dns.OSConfig) (err error)
 		// successfully. See tailscale/tailscale#18679.
 		//
 		// TODO(nickkhyl): revisit and simplify the [multiTUN] implementation?
-		defer func() {
-			if !hasTunnel && b.devices.Down() {
-				b.logger.Logf("updateTUN: tunnel brought down: %v", err)
-			}
-		}()
-	}
-
-	if disableTUN {
+		if b.devices.Down() {
+			b.logger.Logf("updateTUN: tunnel brought down: %v", err)
+		}
 		return nil
 	}
 
@@ -249,7 +235,6 @@ func (b *backend) updateTUN(rcfg *router.Config, dcfg *dns.OSConfig) (err error)
 	b.logger.Logf("updateTUN: created TUN device")
 
 	b.devices.add(tunDev)
-	hasTunnel = true
 	b.logger.Logf("updateTUN: added TUN device")
 
 	if b.devices.Up() {