Import //fetch from Web Platform Tests.

third_party/WebKit/LayoutTests/external/wpt/fetch/api/credentials/authentication-basic.js

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("../resources/utils.js");
+}
+
+function basicAuth(desc, user, pass, mode, status) {
+  promise_test(function(test) {
+    var headers = { "Authorization": "Basic " + btoa(user + ":" + pass)};
+    var requestInit = {"credentials": mode, "headers": headers};
+    return fetch(RESOURCES_DIR + "authentication.py?realm=test", requestInit).then(function(resp) {
+        assert_equals(resp.status, status, "HTTP status is " + status);
+        assert_equals(resp.type , "basic", "Response's type is basic");
+    });
+  }, desc);
+}
+
+basicAuth("User-added Authorization header with include mode", "user", "password", "include", 200);
+basicAuth("User-added Authorization header with same-origin mode", "user", "password", "same-origin", 200);
+basicAuth("User-added Authorization header with omit mode", "user", "password", "omit", 200);
+
+done();