net/http: update Response.Request.URL after redirects on GOOS=js

Fixes #71346

Change-Id: Id4053626e621faf50bb88a10ca0d540f393c8e01
Reviewed-on: https://go-review.googlesource.com/c/go/+/720860
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

src/net/http/roundtrip_js.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"net/http/internal/ascii"
+	"net/url"
 	"strconv"
 	"strings"
 	"syscall/js"
@@ -195,6 +196,13 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 			uncompressed = true
 		}
 
+		if result.Get("redirected").Bool() {
+			u, err := url.Parse(result.Get("url").String())
+			if err == nil {
+				req = req.Clone(req.ctx)
+				req.URL = u
+			}
+		}
 		respCh <- &Response{
 			Status:        fmt.Sprintf("%d %s", code, StatusText(code)),
 			StatusCode:    code,