Revert of Quit the message loop by default in ShellConnectionLost when ApplicationRunner is used

ash/mus/sysui_application.cc

Index: ash/mus/sysui_application.cc
diff --git a/ash/mus/sysui_application.cc b/ash/mus/sysui_application.cc
index 16585a31d4abb7d0c51b6f52f9f233df224449e6..15a518612c845e3ed3cdc7b9a57b26678c722528 100644
--- a/ash/mus/sysui_application.cc
+++ b/ash/mus/sysui_application.cc
@@ -16,6 +16,7 @@
 #include "ash/shell_window_ids.h"
 #include "base/bind.h"
 #include "base/files/file_path.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "components/mus/public/cpp/property_type_converters.h"
@@ -288,5 +289,9 @@
   return true;
 }
 
+void SysUIApplication::ShellConnectionLost() {
+  base::MessageLoop::current()->QuitWhenIdle();
+}
+
 }  // namespace sysui
 }  // namespace ash