MacViews: Style BUTTON_STYLE buttons using the "modern" UI

ui/native_theme/native_theme_mac.h

Index: ui/native_theme/native_theme_mac.h
diff --git a/ui/native_theme/native_theme_mac.h b/ui/native_theme/native_theme_mac.h
index 43071749bb7636f30a85700607813c3f6dd9bbe0..ed57feef8d79ef646b068dc16ea19795388cb818 100644
--- a/ui/native_theme/native_theme_mac.h
+++ b/ui/native_theme/native_theme_mac.h
@@ -16,6 +16,11 @@ class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
  public:
   static const int kComboboxCornerRadius = 5;
 
+  // Type of gradient to use on a button background. Use HIGHLIGHTED for the
+  // default button of a window and all combobox controls, but only when the
+  // window is active.
+  enum class BackgroundType { DISABLED, HIGHLIGHTED, NORMAL, PRESSED, COUNT };
+
   static NativeThemeMac* instance();
 
   // Overridden from NativeTheme:
@@ -33,7 +38,8 @@ class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
       const MenuItemExtraParams& menu_item) const override;
 
   // Creates a shader appropriate for painting the background of a button.
-  static sk_sp<SkShader> GetButtonBackgroundShader(State state, int height);
+  static sk_sp<SkShader> GetButtonBackgroundShader(BackgroundType type,
+                                                   int height);
 
  private:
   NativeThemeMac();