Bug 1664873 - Add `doh-rollout` prefs to "important modified prefs" list in about:support. r=valentin

Differential Revision: https://phabricator.services.mozilla.com/D90221

toolkit/modules/Troubleshoot.jsm

@@ -61,6 +61,7 @@ const PREFS_WHITELIST = [
   "browser.tabs.",
   "browser.urlbar.",
   "browser.zoom.",
+  "doh-rollout.",
   "dom.",
   "extensions.checkCompatibility",
   "extensions.formautofill.",