Handle open URL in DaysUntilBirthday sample app (google#130)

Author: mdmathias

Samples/Swift/DaysUntilBirthday/Shared/DaysUntilBirthday.swift

@@ -37,6 +37,9 @@ struct DaysUntilBirthday: App {
             }
           }
         }
+        .onOpenURL { url in
+          GIDSignIn.sharedInstance.handle(url)
+        }
     }
   }
 }