ash: Instantiate UserActivityForwarder for --mus.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 8cdfec9c861977a5c88ca323d51d7546de3f1125..d85b142b488bfd0633561feda47a253d3c906dbf 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -983,8 +983,10 @@ void Shell::Init(const ShellInitParams& init_params) {
   power_button_controller_->OnDisplayModeChanged(
       display_configurator_->cached_displays());
 
+  // Forward user activity from the window server to |user_activity_detector_|.
   // The connector is unavailable in some tests.
-  if (config == Config::MASH && shell_delegate_->GetShellConnector()) {
+  if (aura::Env::GetInstance()->mode() == aura::Env::Mode::MUS &&
+      shell_delegate_->GetShellConnector()) {
     ui::mojom::UserActivityMonitorPtr user_activity_monitor;
     shell_delegate_->GetShellConnector()->BindInterface(ui::mojom::kServiceName,
                                                         &user_activity_monitor);