Add the button style for combobox

ui/views/controls/button/label_button.h

Index: ui/views/controls/button/label_button.h
diff --git a/ui/views/controls/button/label_button.h b/ui/views/controls/button/label_button.h
index 05dc128861c237d6f80404028b4447e1eeed8b6d..ad7e226ad5e22e11f8f85bcf34f61280b66563c6 100644
--- a/ui/views/controls/button/label_button.h
+++ b/ui/views/controls/button/label_button.h
@@ -23,6 +23,9 @@ class Painter;
 class VIEWS_EXPORT LabelButton : public CustomButton,
                                  public NativeThemeDelegate {
  public:
+  // The length of the hover fade animation.
+  static const int kHoverAnimationDurationMs;
+
   static const char kViewClassName[];
 
   LabelButton(ButtonListener* listener, const string16& text);