Enable lost URL detection in the Physical Web scanner

ios/chrome/common/physical_web/physical_web_scanner.h

 // Copyright 2015 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.
 
 #ifndef IOS_CHROME_COMMON_PHYSICAL_WEB_PHYSICAL_WEB_SCANNER_H_
 #define IOS_CHROME_COMMON_PHYSICAL_WEB_PHYSICAL_WEB_SCANNER_H_
 
 #import <Foundation/Foundation.h>
 
 #include <memory>
 
 namespace base {
 class ListValue;
 }
 
 @protocol PhysicalWebScannerDelegate;
 
 // This class will scan for physical web devices.
 
 @interface PhysicalWebScanner : NSObject
 
+// When onLostDetectionEnabled is YES, the delegate will be notified if a URL
+// that was previously nearby is no longer detected. Changing this value while
+// scanning is active will reset the list of nearby URLs.
+@property(nonatomic, assign) BOOL onLostDetectionEnabled;
+
 // When networkRequest is NO, no network request will be sent.
 @property(nonatomic, assign) BOOL networkRequestEnabled;
 
 // When networkRequest is NO, it returns the number of discovered beacons.
 @property(nonatomic, readonly) int unresolvedBeaconsCount;
 
 // Whether bluetooth is enabled. Must not be read before the first call of
 // scannerBluetoothStatusUpdated.
 @property(nonatomic, readonly) BOOL bluetoothEnabled;
 
@@ skipping 27 matching lines @@
 // This delegate method is called when the list of discovered devices is updated
 // by the scanner.
 - (void)scannerUpdatedDevices:(PhysicalWebScanner*)scanner;
 
 // This delegate method is called when the bluetooth status is updated.
 - (void)scannerBluetoothStatusUpdated:(PhysicalWebScanner*)scanner;
 
 @end
 
 #endif  // IOS_CHROME_COMMON_PHYSICAL_WEB_PHYSICAL_WEB_SCANNER_H_