MacViews: Style BUTTON_STYLE buttons using the "modern" UI

ui/views/style/mac/dialog_button_border_mac.h

Index: ui/views/style/mac/dialog_button_border_mac.h
diff --git a/ui/views/style/mac/dialog_button_border_mac.h b/ui/views/style/mac/dialog_button_border_mac.h
index a45bc48e574c3faa9778e04a4144c395c577dd23..8e639d5c0a65b7c36bf85c5093077d2358076231 100644
--- a/ui/views/style/mac/dialog_button_border_mac.h
+++ b/ui/views/style/mac/dialog_button_border_mac.h
@@ -11,11 +11,16 @@
 
 namespace views {
 
+class CustomButton;
+
 // Skia port of the default button style used for dialogs on Chrome Mac.
 // Originally provided by ConstrainedWindowButton, which used Quartz-backed
 // Cocoa drawing routines.
 class VIEWS_EXPORT DialogButtonBorderMac : public LabelButtonBorder {
  public:
+  // Whether the given |button| should get a highlighted background.
+  static bool ShowsDefault(const CustomButton& button);
+
   DialogButtonBorderMac();
   ~DialogButtonBorderMac() override;