This implements the required plumbing for the keep-alive flag on
requests. The flag already exists on the request object, but now also
exists on the builder itself.
The flag is then passed into a canceller. While we could make canceller
optional, in many cases it isn't and would require a whole lot more
changes. To follow the spec more closely, opted to put the keep_alive
flag on the canceller as well.
Note that this doesn't pass any extra fetch-later tests since we are not
correctly unloading iframe documents. That will be fixed in a follow-up
PR.
Part of #41230
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>