Persistent settings reliability fix (#3512)

Forces a write to always happen when settings are validated. Call to
PendingWrite doesn't by itself do that, if there were no changes
pending.

b/305057554

cobalt/persistent_storage/persistent_settings.cc

@@ -86,6 +86,8 @@ void PersistentSettings::Validate() {
     return;
   }
   validated_initial_settings_ = true;
+  // Report a dummy value as "dirty", so that commit actually writes
+  pref_store_->ReportValueChanged("", 0);
   pref_store_->CommitPendingWrite();
 }