ash uber tray: Make the tray background rounded.

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 8bdd0d493913a3c8bb31f2c0b69e85138b523b64..9aac138b993c61b3ead122ea9730c3558f382ad1 100644
--- a/ash/system/tray/system_tray.h
+++ b/ash/system/tray/system_tray.h
@@ -48,6 +48,8 @@ class ASH_EXPORT SystemTray : public views::View,
 
   // Overridden from views::View.
   virtual bool OnMousePressed(const views::MouseEvent& event) OVERRIDE;
+  virtual void OnMouseEntered(const views::MouseEvent& event) OVERRIDE;
+  virtual void OnMouseExited(const views::MouseEvent& event) OVERRIDE;
 
   // Overridden from views::Widget::Observer.
   virtual void OnWidgetClosing(views::Widget* widget) OVERRIDE;