cros: Set quiet mode for message center during demo sessions

Demo mode is mainly implemented in //chrome, but there're some
initialization needed in //ash. Currently they are placed in
|SessionController::SetIsDemoSession|, we should consider creating
a demo_mode_controller in //ash if the logic becomes complex later.

Bug: 882700
Test: Manual, with/without Mash feature
Change-Id: I945d003875b0cf3864f5796810d09c83e30b76ea
Reviewed-on: https://chromium-review.googlesource.com/1220179Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590499}

ash/session/session_controller.cc

@@ -36,6 +36,7 @@
 #include "services/preferences/public/cpp/pref_service_factory.h"
 #include "services/preferences/public/mojom/preferences.mojom.h"
 #include "services/service_manager/public/cpp/connector.h"
+#include "ui/message_center/message_center.h"
 
 using session_manager::SessionState;
 
@@ -497,6 +498,8 @@ void SessionController::SetIsDemoSession() {
 
   is_demo_session_ = true;
   Shell::Get()->metrics()->StartDemoSessionMetricsRecording();
+  // Notifications should be silenced during demo sessions.
+  message_center::MessageCenter::Get()->SetQuietMode(true);
 }
 
 void SessionController::SetSessionState(SessionState state) {