net/http: add support for SameSite=None
Section 4.2 of the Internet-Draft for SameSite includes the possible
SameSite value of "None".
https://tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00
Change-Id: I44f246024429ec175db13ff6b36bee465f3d233d
GitHub-Last-Rev:
170d24aaca4f00d750fca88740100f7c0b440d19
GitHub-Pull-Request: golang/go#31842
Reviewed-on: https://go-review.googlesource.com/c/go/+/175337
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>