Use mojo SystemTray interfaces for both mash and classic ash

ash/mus/window_manager_application.cc

Index: ash/mus/window_manager_application.cc
diff --git a/ash/mus/window_manager_application.cc b/ash/mus/window_manager_application.cc
index 00cdd43f21afd6152ed5ec69a038ab41155f48d3..d79d19dc77fb61edf7c1506f05025b1316a4b839 100644
--- a/ash/mus/window_manager_application.cc
+++ b/ash/mus/window_manager_application.cc
@@ -7,11 +7,11 @@
 #include <utility>
 
 #include "ash/common/material_design/material_design_controller.h"
+#include "ash/common/system/tray/system_tray_controller.h"
 #include "ash/common/wm_shell.h"
 #include "ash/mus/accelerators/accelerator_registrar_impl.h"
 #include "ash/mus/native_widget_factory_mus.h"
 #include "ash/mus/shelf_delegate_mus.h"
-#include "ash/mus/system_tray_delegate_mus.h"
 #include "ash/mus/wallpaper_delegate_mus.h"
 #include "ash/mus/window_manager.h"
 #include "base/bind.h"
@@ -176,7 +176,8 @@ void WindowManagerApplication::Create(const shell::Identity& remote_identity,
 
 void WindowManagerApplication::Create(const shell::Identity& remote_identity,
                                       mojom::SystemTrayRequest request) {
-  mojom::SystemTray* system_tray = SystemTrayDelegateMus::Get();
+  // Look mom, no cast!
+  mojom::SystemTray* system_tray = WmShell::Get()->system_tray_controller();
   DCHECK(system_tray);
   system_tray_bindings_.AddBinding(system_tray, std::move(request));
 }