Adds a magic header value that is translated to the
Fetch API redirect option, following existing practices.

Updates #26769

Change-Id: Iaf1c9f710de63ea941a360b73f1b4bb725331a35
Reviewed-on: https://go-review.googlesource.com/c/go/+/164666Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>