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 e6916176bf4de6958f2518b8231de018547dd145..1875ced3253132a54d459017cd6074fd1e9ff129 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -52,6 +52,26 @@ class VIEWS_EXPORT PlatformStyle {
   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();
+
+  // Create the appropriate border for a MenuButton.
+  static std::unique_ptr<FocusableBorder> CreateMenuButtonBorder();
+
+  // A combobox or menubutton looks like this:
+  // ------------------------------------
+  // | text field ............. | arrow |
+  // ------------------------------------
+  // These part containing the arrow, *including* any padding around the arrow
+  // or background on the arrow, is called the "shoulder" of the combobox or
+  // menubutton. On Mac, the shoulder has a background, so the text part must
+  // not encroach into it.
+  static int GetComboboxShoulderWidth(Combobox::Style style);
+  static int GetMenuButtonShoulderWidth();
+
   // Creates the default scrollbar for the given orientation.
   static std::unique_ptr<ScrollBar> CreateScrollBar(bool is_horizontal);