ash: Instantiate UserActivityForwarder for --mus.

This class forwards user activity from ws to the
pre-servicification activity-handling mechanism, so it needs
to be used for --mus in addition to --mash. Check for
aura::Env::Mode::MUS to handle both modes.

BUG=717681
TEST=put --mus, --mash, and neither in /etc/chrome_dev.conf
     and check that user activity is reported in
     /var/log/power_manager/powerd.LATEST after hitting a
     key

Review-Url: https://codereview.chromium.org/2863623005
Cr-Commit-Position: refs/heads/master@{#469546}

ash/shell.cc

@@ -982,8 +982,10 @@ 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);