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..ac2a08392c7a93b7393e3884568e2ea2d3d2b490 100644
--- a/ios/chrome/common/physical_web/physical_web_scanner.h
+++ b/ios/chrome/common/physical_web/physical_web_scanner.h
@@ -19,6 +19,10 @@ 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.
+@property(nonatomic, assign) BOOL onLostDetectionEnabled;
+
 // When networkRequest is NO, no network request will be sent.
 @property(nonatomic, assign) BOOL networkRequestEnabled;