Reland: mash: Convert TrayBackgroundView to wm common types

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 064dda51450d535872cdc829f55f1d10383943b7..e733fb61a05af1371b8b28220f6c2d7b95977fde 100644
--- a/ash/system/tray/system_tray.h
+++ b/ash/system/tray/system_tray.h
@@ -10,11 +10,10 @@
 #include <vector>
 
 #include "ash/ash_export.h"
+#include "ash/common/system/tray/system_tray_bubble.h"
+#include "ash/common/system/tray/tray_background_view.h"
 #include "ash/system/cast/tray_cast.h"
-#include "ash/system/tray/system_tray_bubble.h"
-#include "ash/system/tray/tray_background_view.h"
 #include "ash/system/user/login_status.h"
-#include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
 #include "ui/views/bubble/tray_bubble_view.h"
@@ -22,6 +21,7 @@
 
 namespace ash {
 class ScreenTrayItem;
+class StatusAreaWidget;
 class SystemBubbleWrapper;
 class SystemTrayDelegate;
 class SystemTrayItem;
@@ -213,6 +213,9 @@ class ASH_EXPORT SystemTray : public TrayBackgroundView,
   // Overridden from ActionableView.
   bool PerformAction(const ui::Event& event) override;
 
+  // The widget containing this view.
+  StatusAreaWidget* status_area_widget_;
+
   // Owned items.
   ScopedVector<SystemTrayItem> items_;