cros: Adds ShellPort functions for simplified display mode

ash/mus/bridge/shell_port_mash.cc

Index: ash/mus/bridge/shell_port_mash.cc
diff --git a/ash/mus/bridge/shell_port_mash.cc b/ash/mus/bridge/shell_port_mash.cc
index 1a78a0a5a967c52721d8d8517f954e5de6efa541..9973fee2e3f8707d6e6922bd121f8314e180e8d1 100644
--- a/ash/mus/bridge/shell_port_mash.cc
+++ b/ash/mus/bridge/shell_port_mash.cc
@@ -54,6 +54,7 @@
 #include "ui/aura/window.h"
 #include "ui/display/manager/managed_display_info.h"
 #include "ui/display/screen.h"
+#include "ui/display/types/native_display_delegate.h"
 #include "ui/views/mus/pointer_watcher_event_router.h"
 
 namespace ash {
@@ -423,6 +424,11 @@ void ShellPortMash::CreatePointerWatcherAdapter() {
   }
 }
 
+AshWindowTreeHost* ShellPortMash::CreateAshWindowTreeHost(
+    const AshWindowTreeHostInitParams& init_params) {
+  return nullptr;
+}
+
 void ShellPortMash::CreatePrimaryHost() {}
 
 void ShellPortMash::InitHosts(const ShellInitParams& init_params) {
@@ -430,6 +436,11 @@ void ShellPortMash::InitHosts(const ShellInitParams& init_params) {
       base::WrapUnique(init_params.primary_window_tree_host));
 }
 
+std::unique_ptr<display::NativeDisplayDelegate>
+ShellPortMash::CreateNativeDisplayDelegate() {
+  return nullptr;
+}
+
 std::unique_ptr<AcceleratorController>
 ShellPortMash::CreateAcceleratorController() {
   if (GetAshConfig() == Config::MUS) {