Import //fetch from Web Platform Tests.

third_party/WebKit/LayoutTests/external/wpt/fetch/api/redirect/redirect-mode.js

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("/common/get-host-info.sub.js")
+}
+
+function redirectMode(desc, redirectUrl, redirectLocation, redirectStatus, redirectMode) {
+  var url = redirectUrl;
+  var urlParameters = "?redirect_status=" + redirectStatus;
+  urlParameters += "&location=" + encodeURIComponent(redirectLocation);
+
+  var requestInit = {"redirect": redirectMode};
+
+  promise_test(function(test) {
+    if (redirectMode === "error")
+      return promise_rejects(test, new TypeError(), fetch(url + urlParameters, requestInit));
+    if (redirectMode === "manual")
+      return fetch(url + urlParameters, requestInit).then(function(resp) {
+        assert_equals(resp.status, 0, "Response's status is 0");
+        assert_equals(resp.type, "opaqueredirect", "Response's type is opaqueredirect");
+        assert_equals(resp.statusText, "", "Response's statusText is \"\"");
+        assert_equals(resp.url, url + urlParameters, "Response URL should be the original one");
+      });
+    if (redirectMode === "follow")
+      return fetch(url + urlParameters, requestInit).then(function(resp) {
+        assert_true(new URL(resp.url).pathname.endsWith(locationUrl), "Response's url should be the redirected one");
+        assert_equals(resp.status, 200, "Response's status is 200");
+      });
+    assert_unreached(redirectMode + " is no a valid redirect mode");
+  }, desc);
+}
+
+var redirUrl = get_host_info().HTTP_ORIGIN + "/fetch/api/resources/redirect.py";
+var locationUrl = "top.txt";
+
+for (var statusCode of [301, 302, 303, 307, 308]) {
+  redirectMode("Redirect " + statusCode + " in \"error\" mode ", redirUrl, locationUrl, statusCode, "error");
+  redirectMode("Redirect " + statusCode + " in \"follow\" mode ", redirUrl, locationUrl, statusCode, "follow");
+  redirectMode("Redirect " + statusCode + " in \"manual\" mode ", redirUrl, locationUrl, statusCode, "manual");
+}
+
+done();