Enable lost URL detection in the Physical Web scanner

ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm

Index: ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm
diff --git a/ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm b/ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm
index 41ce75763a7144104d88c660a349203692f862c8..12edafa0a6d3178384cd40a280c991762cbeffec 100644
--- a/ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm
+++ b/ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm
@@ -26,6 +26,9 @@ void IOSChromePhysicalWebDataSource::StartDiscovery(
 
   [scanner_ stop];
   scanner_.reset([[PhysicalWebScanner alloc] initWithDelegate:nil]);
+
+  // Configure the scanner to notify us when a URL is no longer nearby.
+  [scanner_ setOnLostDetectionEnabled:YES];
   [scanner_ setNetworkRequestEnabled:network_request_enabled];
   [scanner_ start];
 }