mash: Convert TrayBackgroundView to wm common types

ash/system/overview/overview_button_tray.h

Index: ash/system/overview/overview_button_tray.h
diff --git a/ash/system/overview/overview_button_tray.h b/ash/system/overview/overview_button_tray.h
index 8ed128fac7be70ca5c65efe28842d18484af60d5..4df6b00374e647dd88f6335f7c76c73f781c7ae3 100644
--- a/ash/system/overview/overview_button_tray.h
+++ b/ash/system/overview/overview_button_tray.h
@@ -8,7 +8,7 @@
 #include "ash/ash_export.h"
 #include "ash/common/session/session_state_observer.h"
 #include "ash/common/shell_observer.h"
-#include "ash/system/tray/tray_background_view.h"
+#include "ash/common/system/tray/tray_background_view.h"
 #include "base/macros.h"
 
 namespace views {
@@ -17,6 +17,8 @@ class ImageView;
 
 namespace ash {
 
+class StatusAreaWidget;
+
 // Status area tray for showing a toggle for Overview Mode. Overview Mode
 // is equivalent to WindowSelectorController being in selection mode.
 // This hosts a ShellObserver that listens for the activation of Maximize Mode