Add fallback feature detection for Array.prototype.findLast

src/html/_js_base.html.template

@@ -16,5 +16,9 @@
   ) {
     _ls("/static/polyfills/webcomponents-bundle.js", true);
   }
-  var isModern = <%= modernRegex %>.test(navigator.userAgent);
+  // Modern browsers are detected primarily using the user agent string.
+  // A feature detection which roughly lines up with the modern targets is used
+  // as a fallback to guard against spoofs. It should be updated periodically.
+  var isModern = <%= modernRegex %>.test(navigator.userAgent) &&
+    "findLast" in Array.prototype;
 </script>