Bug 1651120 - Add cookie sameSite schemeful as an experimental feature, r=Gijs,fluent-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D82561

toolkit/components/featuregates/Features.toml

@@ -58,3 +58,13 @@ type = "boolean"
 bug-numbers = [1614351]
 is-public = true
 default-value = false
+
+[cookie-samesite-schemeful]
+title = "experimental-features-cookie-samesite-schemeful"
+description = "experimental-features-cookie-samesite-schemeful-description"
+restart-required = false
+preference = "network.cookie.sameSite.schemeful"
+type = "boolean"
+bug-numbers = [1651119]
+is-public = true
+default-value = {default = false, nightly = true}

toolkit/locales/en-US/toolkit/featuregates/features.ftl

@@ -35,3 +35,9 @@ experimental-features-cookie-samesite-none-requires-secure2-description = Cookie
 experimental-features-abouthome-startup-cache =
     .label = about:home startup cache
 experimental-features-abouthome-startup-cache-description = A cache for the initial about:home document that is loaded by default at startup. The purpose of the cache is to improve startup performance.
+
+# The title of the experiment should be kept in English as it may be referenced
+# by various online articles and is technical in nature.
+experimental-features-cookie-samesite-schemeful =
+    .label = Cookies: Schemeful SameSite
+experimental-features-cookie-samesite-schemeful-description = Treat cookies from the same domain, but with different schemes (e.g. http://example.com and https://example.com) as cross-site instead of same-site. Improves security, but potentially introduces breakage.

toolkit/modules/tests/browser/browser_Troubleshoot.js

@@ -42,6 +42,7 @@ var tests = [
         set: [
           ["network.cookie.sameSite.laxByDefault", false],
           ["network.cookie.sameSite.noneRequiresSecure", false],
+          ["network.cookie.sameSite.schemeful", false],
         ],
       },
       done