Delay generation of User-Agent header to URLRequestHttpJob and accept custom User-Agent from XHR/Fe…

third_party/WebKit/LayoutTests/http/tests/fetch/resources/useragent-header.js

+function parse_response(body) {
+  const lines = body.split('\n');
+  for (let line of lines) {
+    if (line.length == 0) {
+      continue;
+    }
+    const parts = line.split(': ', 2);
+    if (parts.length < 2) {
+      return 'ERROR: Invalid line in response: ' + line;
+    }
+    if (parts[0] == 'HTTP_USER_AGENT') {
+      return parts[1];
+    }
+  }
+  return 'ERROR: User-Agent header not found';
+}