Bug 1650790 - Add the about:home startup cache to the about:preferences experiments section. r=Gijs,fluent-reviewers

The cache is still disabled by default, but it would be good to collect more bugs and feedback early.

Differential Revision: https://phabricator.services.mozilla.com/D82394

browser/components/preferences/tests/browser_search_subdialogs_within_preferences_site_data.js

@@ -36,7 +36,10 @@ add_task(async function() {
   await openPreferencesViaOpenPreferencesAPI("paneGeneral", {
     leaveOpen: true,
   });
-  await evaluateSearchResults("cache", ["siteDataGroup"]);
+  await evaluateSearchResults("cache", [
+    "siteDataGroup",
+    "pane-experimental-featureGates",
+  ]);
   BrowserTestUtils.removeTab(gBrowser.selectedTab);
 });
 

toolkit/components/featuregates/Features.toml

@@ -48,3 +48,13 @@ type = "boolean"
 bug-numbers = [1618610]
 is-public = true
 default-value = {default = false, nightly = true}
+
+[abouthome-startup-cache]
+title = "experimental-features-abouthome-startup-cache"
+description = "experimental-features-abouthome-startup-cache-description"
+restart-required = true
+preference = "browser.startup.homepage.abouthome_cache.enabled"
+type = "boolean"
+bug-numbers = [1614351]
+is-public = true
+default-value = false

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

@@ -31,3 +31,9 @@ experimental-features-cookie-samesite-lax-by-default-description = Treat cookies
 experimental-features-cookie-samesite-none-requires-secure =
     .label = Cookies: SameSite=none requires secure attribute
 experimental-features-cookie-samesite-none-requires-secure-description = Cookies with “sameSite=none” attribute require the secure attribute. This feature requires “Cookies: SameSite=lax by default”.
+
+# about:home should be kept in English, as it refers to the the URI for
+# the internal default home page.
+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.