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

ash/mus/sysui_application.h

Index: ash/mus/sysui_application.h
diff --git a/ash/mus/sysui_application.h b/ash/mus/sysui_application.h
index dba0f5e3dc28d611a985b4767c302f017fa3b96b..2535c5e08b4c22864eed8cd033e4eca0ce1dff69 100644
--- a/ash/mus/sysui_application.h
+++ b/ash/mus/sysui_application.h
@@ -26,6 +26,7 @@
                   const mojo::Identity& identity,
                   uint32_t id) override;
   bool AcceptConnection(mojo::Connection* connection) override;
+  void ShellConnectionLost() override;
 
   mojo::TracingImpl tracing_;
   scoped_ptr<AshInit> ash_init_;