MenuButton: support Mac look & feel

ui/views/style/platform_style.h

Index: ui/views/style/platform_style.h
diff --git a/ui/views/style/platform_style.h b/ui/views/style/platform_style.h
index 97f697e4fd4e2f2e4e9e41137453c1b8601fbaca..c3f61377c69a1caeb6906ac7aca299c5bee47017 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -49,13 +49,24 @@ class VIEWS_EXPORT PlatformStyle {
   static std::unique_ptr<FocusableBorder> CreateComboboxBorder();
 
   // Creates the appropriate background for a Combobox.
-  static std::unique_ptr<Background> CreateComboboxBackground();
+  static std::unique_ptr<Background> CreateComboboxBackground(
+      int shoulder_width);
 
   // Creates the default label button border for the given |style|. Used when a
   // custom default border is not provided for a particular LabelButton class.
   static std::unique_ptr<LabelButtonBorder> CreateLabelButtonBorder(
       Button::ButtonStyle style);
 
+  // Creates an ImageSkia containing the image to use for the menubutton arrow.
+  static gfx::ImageSkia CreateMenuButtonArrow(bool is_enabled);
+
+  // Create the appropriate background for a MenuButton.
+  static std::unique_ptr<Background> CreateMenuButtonBackground(
+      int shoulder_width);
+
+  // Create the appropriate border for a MenuButton.
+  static std::unique_ptr<FocusableBorder> CreateMenuButtonBorder();
+
   // Creates the default scrollbar for the given orientation.
   static std::unique_ptr<ScrollBar> CreateScrollBar(bool is_horizontal);