Enable lost URL detection in the Physical Web scanner

ios/chrome/common/physical_web/physical_web_scanner.h

Index: ios/chrome/common/physical_web/physical_web_scanner.h
diff --git a/ios/chrome/common/physical_web/physical_web_scanner.h b/ios/chrome/common/physical_web/physical_web_scanner.h
index c147bc29bca7b3785ab8b5817e0fec4374e99c7b..8873f3bf23535af4f0ffdf76930ed982b6a99c9f 100644
--- a/ios/chrome/common/physical_web/physical_web_scanner.h
+++ b/ios/chrome/common/physical_web/physical_web_scanner.h
@@ -19,6 +19,11 @@ class ListValue;
 
 @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;