Use mojo SystemTray interfaces for both mash and classic ash

chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc

Index: chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc
diff --git a/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc b/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc
index 704550cb08997c82340569bfa73356b48ccc3a21..2384ef180805d337375a8e9701238dbbb01dc3ce 100644
--- a/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc
+++ b/chrome/browser/ui/views/ash/chrome_browser_main_extra_parts_ash.cc
@@ -41,6 +41,7 @@ void ChromeBrowserMainExtraPartsAsh::PreProfileInit() {
     immersive_handler_factory_ = base::MakeUnique<ImmersiveHandlerFactoryMus>();
 #if defined(OS_CHROMEOS)
     // Must be available at login screen, so initialize before profile.
+    // TODO: Always create this, even when not in mash.
     system_tray_client_ = base::MakeUnique<SystemTrayClient>();
 #endif
   }