Enable lost URL detection in the Physical Web scanner

ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/values.h"
 #import "ios/chrome/common/physical_web/physical_web_scanner.h"
 
 IOSChromePhysicalWebDataSource::IOSChromePhysicalWebDataSource() {}
 
 IOSChromePhysicalWebDataSource::~IOSChromePhysicalWebDataSource() {
   StopDiscovery();
 }
 
 void IOSChromePhysicalWebDataSource::StartDiscovery(
     bool network_request_enabled) {
   // If there are unresolved beacons it means the scanner is started but does
   // not have network requests enabled. In this case we should avoid recreating
   // the scanner as it would clear the cache of nearby beacons.
   if (network_request_enabled && HasUnresolvedDiscoveries()) {
     [scanner_ setNetworkRequestEnabled:YES];
     return;
   }
 
   [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];
 }
 
 void IOSChromePhysicalWebDataSource::StopDiscovery() {
   [scanner_ stop];
   scanner_.reset();
 }
 
 std::unique_ptr<base::ListValue> IOSChromePhysicalWebDataSource::GetMetadata() {
   if (!scanner_) {
     return base::MakeUnique<base::ListValue>();
   }
   return [scanner_ metadata];
 }
 
 bool IOSChromePhysicalWebDataSource::HasUnresolvedDiscoveries() {
   return [scanner_ unresolvedBeaconsCount] > 0;
 }