Bug 1541233 - Don't load customization style sheets if toolkit.legacyUserProfileCustomizations.stylesheets pref is not set. r=mconley,emilio

Differential Revision: https://phabricator.services.mozilla.com/D31670

--HG--
extra : moz-landing-system : lando

browser/base/content/test/performance/browser_startup_mainthreadio.js

@@ -289,16 +289,6 @@ const startupPhases = {
       read: 3,
       close: 3,
     },
-    {
-      path: "UChrm:userChrome.css",
-      condition: WIN,
-      stat: 1,
-    },
-    { // bug 1541233
-      path: "UChrm:userContent.css",
-      condition: WIN,
-      stat: 1,
-    },
     { // bug 1541246
       path: "XREUSysExt:",
       condition: WIN,

layout/style/nsLayoutStylesheetCache.cpp

@@ -416,6 +416,10 @@ nsLayoutStylesheetCache* nsLayoutStylesheetCache::Singleton() {
 }
 
 void nsLayoutStylesheetCache::InitFromProfile() {
+  if (!Preferences::GetBool(PREF_LEGACY_STYLESHEET_CUSTOMIZATION)) {
+    return;
+  }
+
   nsCOMPtr<nsIXULRuntime> appInfo =
       do_GetService("@mozilla.org/xre/app-info;1");
   if (appInfo) {
@@ -444,21 +448,6 @@ void nsLayoutStylesheetCache::InitFromProfile() {
                 eLogToConsole);
 
   if (XRE_IsParentProcess()) {
-    if (mUserChromeSheet || mUserContentSheet) {
-      // Bug 1541233 aims to avoid stat'ing or loading userContent.css or
-      // userChrome.css during start-up by default. After that point,
-      // PREF_LEGACY_STYLESHEET_CUSTOMIZATION pref is how users can opt-in
-      // to continuing to use userChrome.css or userContent.css.
-      //
-      // Before bug 1541233 lands though, we'll ship a release which
-      // continues to look for those files on start-up and sets a pref.
-      // That way, in a subsequent release when loading those files is
-      // off by default, those users will still get their userChrome.css
-      // and userContent.css customizations without having to manually
-      // set the pref themselves.
-      Preferences::SetBool(PREF_LEGACY_STYLESHEET_CUSTOMIZATION, true);
-    }
-
     // We're interested specifically in potential chrome customizations,
     // so we only need data points from the parent process
     Telemetry::Accumulate(Telemetry::USER_CHROME_CSS_LOADED,

testing/profiles/reftest/user.js

@@ -103,3 +103,5 @@ user_pref("ui.systemUsesDarkTheme", 0);
 user_pref("dom.push.serverURL", "");
 // Disable intermittent telemetry collection
 user_pref("toolkit.telemetry.initDelay", 99999999);
+// Setting this pref to true for usercss reftests, since it relies on userContent.css
+user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true);