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

api/next.txt

@@ -180,6 +180,8 @@ pkg net, type ListenConfig struct, KeepAlive time.Duration
 pkg net/http, const StatusEarlyHints = 103
 pkg net/http, const StatusEarlyHints ideal-int
 pkg net/http, method (Header) Clone() Header
+pkg net/http, const SameSiteNoneMode = 4
+pkg net/http, const SameSiteNoneMode SameSite
 pkg net/http, type Server struct, BaseContext func(net.Listener) context.Context
 pkg net/http, type Server struct, ConnContext func(context.Context, net.Conn) context.Context
 pkg net/http, type Transport struct, ForceAttemptHTTP2 bool

src/net/http/cookie.go

@@ -48,6 +48,7 @@ const (
 	SameSiteDefaultMode SameSite = iota + 1
 	SameSiteLaxMode
 	SameSiteStrictMode
+	SameSiteNoneMode
 )
 
 // readSetCookies parses all "Set-Cookie" values from
@@ -105,6 +106,8 @@ func readSetCookies(h Header) []*Cookie {
 					c.SameSite = SameSiteLaxMode
 				case "strict":
 					c.SameSite = SameSiteStrictMode
+				case "none":
+					c.SameSite = SameSiteNoneMode
 				default:
 					c.SameSite = SameSiteDefaultMode
 				}
@@ -217,6 +220,8 @@ func (c *Cookie) String() string {
 	switch c.SameSite {
 	case SameSiteDefaultMode:
 		b.WriteString("; SameSite")
+	case SameSiteNoneMode:
+		b.WriteString("; SameSite=None")
 	case SameSiteLaxMode:
 		b.WriteString("; SameSite=Lax")
 	case SameSiteStrictMode:

src/net/http/cookie_test.go

@@ -77,6 +77,10 @@ var writeSetCookiesTests = []struct {
 		&Cookie{Name: "cookie-14", Value: "samesite-strict", SameSite: SameSiteStrictMode},
 		"cookie-14=samesite-strict; SameSite=Strict",
 	},
+	{
+		&Cookie{Name: "cookie-15", Value: "samesite-none", SameSite: SameSiteNoneMode},
+		"cookie-15=samesite-none; SameSite=None",
+	},
 	// The "special" cookies have values containing commas or spaces which
 	// are disallowed by RFC 6265 but are common in the wild.
 	{
@@ -296,6 +300,15 @@ var readSetCookiesTests = []struct {
 			Raw:      "samesitestrict=foo; SameSite=Strict",
 		}},
 	},
+	{
+		Header{"Set-Cookie": {"samesitenone=foo; SameSite=None"}},
+		[]*Cookie{{
+			Name:     "samesitenone",
+			Value:    "foo",
+			SameSite: SameSiteNoneMode,
+			Raw:      "samesitenone=foo; SameSite=None",
+		}},
+	},
 	// Make sure we can properly read back the Set-Cookie headers we create
 	// for values containing spaces or commas:
 	{