MacViews: Style BUTTON_STYLE buttons using the "modern" UI

ui/views/controls/button/custom_button.h

Index: ui/views/controls/button/custom_button.h
diff --git a/ui/views/controls/button/custom_button.h b/ui/views/controls/button/custom_button.h
index d91d3e68771cd9ec6ec873891541b048ec1437a2..c478b9f1dd730d873e4106e0578f22095aa5fff6 100644
--- a/ui/views/controls/button/custom_button.h
+++ b/ui/views/controls/button/custom_button.h
@@ -16,6 +16,7 @@
 namespace views {
 
 class InkDropDelegate;
+class LabelButton;
 
 // A button with custom rendering. The base of ImageButton and LabelButton.
 // Note that this type of button is not focusable by default and will not be
@@ -34,6 +35,8 @@ class VIEWS_EXPORT CustomButton : public Button, public gfx::AnimationDelegate {
 
   static const CustomButton* AsCustomButton(const views::View* view);
   static CustomButton* AsCustomButton(views::View* view);
+  static const LabelButton* AsLabelButton(const views::View* view);
+  static LabelButton* AsLabelButton(views::View* view);
 
   ~CustomButton() override;