Implements a display scale tray setting behind a chrome switch

ash/system/tray/system_tray.h

Index: ash/system/tray/system_tray.h
diff --git a/ash/system/tray/system_tray.h b/ash/system/tray/system_tray.h
index 5e5244d1441954ae5ebddf7339ddf1c5bf2d94d2..018812487857cf06f3e878147c653144863ad6d9 100644
--- a/ash/system/tray/system_tray.h
+++ b/ash/system/tray/system_tray.h
@@ -29,6 +29,7 @@ class TrayAudio;
 class TrayCast;
 class TrayEnterprise;
 class TrayNetwork;
+class TrayScale;
 class TraySupervisedUser;
 class TraySystemInfo;
 class TrayTiles;
@@ -231,6 +232,7 @@ class ASH_EXPORT SystemTray : public TrayBackgroundView,
   TrayEnterprise* tray_enterprise_ = nullptr;
   TrayNetwork* tray_network_ = nullptr;
   TrayTiles* tray_tiles_ = nullptr;
+  TrayScale* tray_scale_ = nullptr;
   TraySupervisedUser* tray_supervised_user_ = nullptr;
   TraySystemInfo* tray_system_info_ = nullptr;
   TrayUpdate* tray_update_ = nullptr;