[test] Convert browser_harness.html to async/await. NFC (#26877)

Author: sbc100

test/browser_harness.html

@@ -23,32 +23,32 @@
     }
   </style>
   <script>
-    var COMMAND_PREFIX = 'COMMAND:';
     var counter = 0;
-    function check() {
-      fetch('/check')
-        .then((rsp) => rsp.text())
-        .then((responseText) => {
-          if (responseText.startsWith(COMMAND_PREFIX)) {
-            var url = responseText.substr(COMMAND_PREFIX.length);
-            iframe.src = url;
-            document.getElementById('count').textContent = counter++;
-          }
-          // Polling for the next test to start: we just keep asking if we
-          // should load a new page, since the iframe doesn't currently have
-          // a way to tell us it completed (even if it did, we'd need to poll
-          // the server to know when the next test page is ready to load).
-          // (A timeout value of 100msecs has been seen to stall test
-          //  browser.test_audio_worklet_worker on Chrome)
-          setTimeout(check, 200);
-        })
-        .catch(() => {
-          document.body.innerHTML = 'Tests complete. View log in console.';
-          // Attempt to close the main window.  This works on chrome
-          // but fails on firefox with: `Scripts may not close windows that
-          // were not opened by script.`
-          window.close();
-        });
+    async function check() {
+      try {
+        const rsp = await fetch('/check');
+        const responseText = await rsp.text();
+
+        const COMMAND_PREFIX = 'COMMAND:';
+        if (responseText.startsWith(COMMAND_PREFIX)) {
+          var url = responseText.substr(COMMAND_PREFIX.length);
+          iframe.src = url;
+          document.getElementById('count').textContent = counter++;
+        }
+        // Polling for the next test to start: we just keep asking if we
+        // should load a new page, since the iframe doesn't currently have
+        // a way to tell us it completed (even if it did, we'd need to poll
+        // the server to know when the next test page is ready to load).
+        // (A timeout value of 100msecs has been seen to stall test
+        //  browser.test_audio_worklet_worker on Chrome)
+        setTimeout(check, 200);
+      } catch {
+        document.body.innerHTML = 'Tests complete. View log in console.';
+        // Attempt to close the main window.  This works on chrome
+        // but fails on firefox with: `Scripts may not close windows that
+        // were not opened by script.`
+        window.close();
+      }
     }
     document.addEventListener('DOMContentLoaded', check);
   </script>