Implements a display scale tray setting behind a chrome switch

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index cc76d1068097efec0d4d5068776e8af061cd296a..83269af57d4427aa022d36a079caf6cc00be2d1c 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -22,6 +22,7 @@
 #include "ash/system/brightness/tray_brightness.h"
 #include "ash/system/cast/tray_cast.h"
 #include "ash/system/date/tray_system_info.h"
+#include "ash/system/display_scale/tray_scale.h"
 #include "ash/system/enterprise/tray_enterprise.h"
 #include "ash/system/ime/tray_ime_chromeos.h"
 #include "ash/system/media_security/multi_profile_media_tray_item.h"
@@ -271,6 +272,8 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
   AddTrayItem(base::MakeUnique<MultiProfileMediaTrayItem>(this));
   tray_audio_ = new TrayAudio(this);
   AddTrayItem(base::WrapUnique(tray_audio_));
+  tray_scale_ = new TrayScale(this);
+  AddTrayItem(base::WrapUnique(tray_scale_));
   AddTrayItem(base::MakeUnique<TrayBrightness>(this));
   AddTrayItem(base::MakeUnique<TrayCapsLock>(this));
   // TODO(jamescook): Remove this when mus has support for display management