Add LayoutTest for the script error sanitization for ServiceWorker fetched scripts.

LayoutTests/http/tests/serviceworker/chromium/resources/fetch-script-onerror-iframe.html

+<script src="../../../resources/get-host-info.js"></script>
+<script>
+var host_info = get_host_info();
+
+window.addEventListener('error', function(evt) {
+    window.parent.postMessage(
+        {
+          filename: evt.filename,
+          colno: evt.colno,
+          lineno: evt.lineno,
+          message: evt.message
+        },
+        host_info['HTTP_ORIGIN']);
+  });
+
+window.addEventListener('message', function(evt) {
+    var script = document.createElement('script');
+    script.src = evt.data.url;
+    document.body.appendChild(script);
+  });
+</script>